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3:07 AM
Angiosperm Cells and Tissues
Angiosperm Cells and Tissues
Some cell types and tissues which are not found in any other groups of plants occur in angiosperms (flowering plants).

Angiosperms are a group of plants with seeds that develop within an ovary and reproductive organs in flowers. They are commonly referred to as flowering plants and represent the most successful group of plants on earth, with approximately 235,000 species.

Various cell types and tissues, many of which are not found in any other groups of plants, occur in angiosperms. These cells and tissues perform varied functions, which are very efficient compared to their counterparts in other plants. These include dermal, vascular (xylem and phloem), and ground tissues (such as parenchyma, collenchyma, and sclerenchyma).

The growth of plants is carried on by a group of cells at their tips. These groups of cells are referred to as apical meristems, which are composed of initials and their most recent derivatives. The initials are the main source of body cells in plants,while the derivatives become any of the cells and tissues in the plant body.

The apical meristems of both the shoot and the root show continued cell division, with cells enlarging, elongating, and differentiating in regular, distinctly organized patterns. Apical meristems bring about the increase in the length of the stems and roots and are responsible in the formation of the primary plant body.

The shoot apical meristem may continually initiate the aerial components of the plant or may enter a state of periodic quiescence. In some plants, the shoot apical meristem transforms into a floral or inflorescence meristem that eventually terminates in a single flower or clusters of flowers, respectively.

The root apical meristem is enclosed by a thimble-shaped root cap that hastens the penetration of roots between soil particles.Unlike the shoot apical meristem, the root apical meristem forms no appendages. In fact, the site of lateral root initiation is far removed from it.

Shoot Apex

The shoot apical meristem is typically dome-shaped but flattened, and concave outlines also exist. The outline is not constant but changes in response to plastochron (the time interval between the initiation of one leaf primordium and the next). At least three models describe the shoot apical meristems. Although each of these is based on one or two unique criteria, they also have a few overlapping features.

Cell Lineage Analysis

This model holds that three clonally related layers of cells characterize the shoot apical meristem. These layers can be more than one cell layer thick. L1 is the outermost layer and gives rise to the epidermis, L2 is the middle layer and gives rise to the vascular tissues and cortex, and L3 is the inner most layer and gives rise to the pith.

This model was based on studies using periclinal chimeras (organs or parts of tissues of diverse genetic constitution), where one of the cell layers was genetically altered using drugs that inhibit separation of chromosomes.

Tunica-Corpus Concept

Tunica-Corpus Concept
Tunica-Corpus Concept
This model is based on microscopic analysis of constituent cells. It says that the shoot apical meristem is made up of two groups of cells. The tunica, a group of cells that form one or two stratified layers, undergoes anticlinal divisions only and gives rise to the epidermis.

Partly enclosed by the tunica is the corpus, a group of loosely arranged cells that divide in various planes and give rise to the vascular and ground tissues. The tunica maintains its individuality by surface growth, whereas the corpus adds bulk by increase in volume.

Cytohistological Zonation

This model recognizes various definable zones in the shoot apical meristem. Three zone boundaries are distinguished by cell size: staining quality, degree of vacuolation, and frequency of cell division.

The central (mother cell) zone represents a conspicuous group of enlarged and isodiametric cells that undergo infrequent cell division, possess prominent nuclei, and are often highly vacuolated. The flanking peripheral zone is derived from, and partly surrounds, the central zone. Cells of this zone are smaller, are mitotically active, and have dense cytoplasms.

They give rise to the epidermis, vascular tissues, and cortex. The rib zone is located at the base of the central and peripheral zones. This zone is directly formed from the central zone, produces longitudinal files of cells by periclinal divisions, and gives rise to the pith.

Root Apex

Root Apex
Root Apex
The organization of the root apical meristem is different from that of the shoot apical meristem. Root apical meristems are commonly interpreted as having either a close or open type of organization. In a close type of organization, the dermal, vascular, and ground tissues each have their own set of initials.

This organization shows a clear boundary between root cap and other tissues of the root apex. In an open type of organization, all of the root tissues share a group of initials, and therefore the boundary of the root cap is indistinguishable from the other tissues of the root apex.

Developmental Processes

The cells produced by apical meristems undergo several key developmental processes, which include growth, differentiation, and morphogenesis. Although each of these can be separated individually, they overlap in highly complex fashion.

Growth refers to the quantitative increase in a cell’s volume or mass due to enlargement and multiplication. Differentiation is the qualitative change in the form and function of organelles, cells, tissues, and organs, resulting in the establishment of new structures and functions.

From an anatomical point of view, cell differentiation is related to changes in cell size and shape, modifications of the wall, and changes in staining characteristics of nucleus or cytoplasm, as well as the degree of vacuolation and the ultimate loss of the protoplast in some cases. Morphogenesis is the visible manifestation of all of the changes, brought about by growth and differentiation, as expressed in the overall morphology of the plant.

Dermal Tissues

The primary plant body is composed of three basic tissues: dermal, vascular, and ground tissues. The dermal tissue (or epidermis) is made up of several cell types and is involved in a variety of functions, including retention and absorption of water and minerals, protection against herbivores, and control of gas exchange. Each of these functions is attributable to one or more of the unique features of the epidermis.

Most epidermal cells are flat and tightly packed, forming a single layer around stems, leaves, and other organs. The outer walls of epidermal cells are equipped with a waterproof layer made up of a fatty material called cutin. The tightly packed and cutinized epidermis protects the plants from desiccation by keeping moisture in.

Epidermal cells lack chloroplasts and are transparent. It is the underlying cells that give leaves and stems their green color. However, the vacuoles of some epidermal cells occasionally contain pigments and are responsible in the coloration of flowers and colored parts of variegated leaves.

Stomata are specialized structures that form part of the epidermis of leaves, stems, flowers, and fruits. They are involved in regulating the intake of carbon dioxide for photosynthesis as well as the release of oxygen. Trichomes are single-celled or multicellular out growths of epidermal cells that are involved in deterring herbivores and restricting transpiration.

Root hairs are also outgrowths of epidermal cells that are specialized for absorbing water and minerals from soil. They occur near the tip of the root and function to increase its absorptive surface area several-thousand fold.

Vascular Tissues

Vascular Tissues
Vascular Tissues
Vascular tissues are of two types: xylem and phloem. Xylem occurs throughout the plant body, and the type that differentiates directly from the apical meristem is called primary xylem. (Secondary xylem is formed from the vascular cambium.)

Primary xylem is formed as stems and roots elongate. The two kinds of conducting cells in xylem are tracheids and vessels, or vessel elements. Both are dead at maturity and have thick, lignified secondary cell walls. Tracheids are long, slender cells with tapered, overlapping ends.

They are the only water-conducting cells in most gymnosperms (an evolutionary line of plants that includes conifers).Water moves upward in roots and stems from tracheid to tracheid through thin areas in their cellwalls called pits. With only a few exceptions, all angiosperms contain vessel elements and tracheids.

Vessel elements are short, wide in diameter, and connected end to end. Their end walls are partly or wholly dissolved, forming long hollow vessels through which water moves. All these features of vessels enable them to transport water more rapidly than tracheids.

Phloem transports dissolved organic materials throughout the plant. The conducting cells of the phloem are called sieve elements, which are devoid of nuclei but otherwise have intact cytoplasm. They also have thin areas along their cell walls called sieve areas that are perforated. Solutes move from sieve element to sieve element through these pores.

Ground Tissues

The three types of ground tissue are parenchyma, collenchyma, and sclerenchyma. Parenchyma cells are the most abundant and versatile cells in plants. These cells are isodiametric, are alive at maturity, are highly vacuolated, and have a primary cell wall. Parenchyma functions as food-and-water-storage tissue as well as sites of metabolism in plants. Chlorenchyma cells are chloroplast-containing parenchyma specialized for photosynthesis.

Collenchyma cells are relatively long, with unevenly thickened primary walls. They support growing regions of shoots and are common in petioles, elongating stems and expanding leaves. Collenchyma cells are well adapted for support because their cell walls are able to stretch. They often form in strands or a cylinder just beneath the epidermis; such location maximizes support, as would a rod located in the center of a stemor petiole (leaf base).

Sclerenchyma cells are rigid; produce thick, non-stretchable secondary walls; and are usually dead atmaturity. They occur in, support, and strengthen mature regions of plants, including stems, roots, and leaves. There are two types of sclerenchyma cells: sclereids and fibers.

Sclereids are relatively short and variable in shape and usually occur in small groups. Fibers are long and slender and occur in strands or bundles. Sclereids are found in the roots, leaves, and stems.

They produce the gritty texture of pears and mostly make up the tough core of apples as well as the seed coats of peanuts and walnuts. Fibers are often associated with vascular tissues and, compared to sclereids, are typically elongated cells that vary in length from a few millimeters to more than half a meter long.
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1:56 AM
Angiosperm
Angiosperm
Angiosperms (flowering plants) appeared about 130 million years ago and today dominate the plant world, with approximately 235,000 species.

In early Devonian-age rocks, approximately 363- 409 million years old, fossils of simple vascular and nonvascular plants can be seen. Ferns, lycopods, horsetails, and early gymnosperms became prominent during the Carboniferous period (approximately 290-363 million years ago).

The gymnosperms were the dominant flora during the Age of Dinosaurs, the Mesozoic era (65-245million years ago). More than 130 million years ago, from the Jurassic period to early in the Cretaceous period, the first flowering plants, or angiosperms (phylum Anthophyta), arose. Over the following 40 million years, angiosperms became the world’s dominant plants.

The angiosperms show high species diversity, and they occupy almost every habitat on earth, from deserts to high mountain peaks and from freshwater ecosystems to marine estuaries. Angiosperms range in size from eucalyptus trees well over 100 meters (328 feet) tall with trunks nearly 20 meters (66 feet) in circumference to duckweed, simple floating plants barely 1 millimeter (0.003 inch) long.


Special Characteristics

Some of the defining characteristics of angiosperms involve their physical appearance or morphology and internal anatomy: the presence of flowers and fruits containing seeds, stamens with two pairs of pollen sacs, a microgametophyte (the male, haploid stage of the life cycle contained in the pollen) with three nuclei, a megagametophyte (the female, haploid stage of the life cycle enclosed in the ovary) with eight nuclei, companion cells, and sieve tubes in the phloem (vascular tissue important in the transport of organic molecules).

Some of these characteristics involve life-cycle features, such as double fertilization, that are distinct from almost all other members of the plant kingdom. (Double fertilization is also known in the genera Ephedra and Gnetum, members of the gymnosperms.)

Because angiosperms possess so many unique features, plant taxonomists have long believed that angiosperms originated from a single common ancestor. Because the first flowers and pollen grains appear in fossils from the early Cretaceous period, up to about 130 million years ago, it is probable that angiosperms actually arose more than 130 million years ago.

As the findings of paleobotanists (botanists who study plants in the fossil record) have been combined with more recent knowledge from evolutionary genetics and biochemistry, a clearer picture of angiosperm evolution has emerged.

Proposed Ancestors

Because gymnosperms (the other large group of seed plants) have long been considered ancestral to the angiosperms, researchers have attempted to develop models for the evolution of the ovule-bearing structures of flowering plants from the similar, naked ovule-bearing structures of gymnosperms.

Angiosperm evolution
Angiosperm evolution

Some lines of evidence indicate that groups of extinct cycad-like gymnosperms known as the Bennettitales and the gnetophytes, a modern division of the gymnosperms which show up in the fossil record about 225 million years ago, are the seed plants most closely related to angiosperms.

All three groups, the Bennettitales, the gnetophytes, and the angiosperms, share, or shared, superficially similar flower-like reproductive structures. The strobili, or cones, of some gnetophytes closely resemble flowers, and the xylem (vascular tissue specialized for transporting water) of some gnetophytes is similar to the xylem found in angiosperms.

Seed Ferns

seed ferns (pteridosperms) fossil
seed ferns (pteridosperms) fossil

Other lines of evidence suggest that a group of plants called the seed ferns, or pteridosperms, might represent the ancestors of the angiosperms. The seed ferns, which predate the angiosperms by many millions of years, had seed-bearing cupules and specialized organs that produced pollen. Many plant taxonomists believe that the seed-bearing cupules in some groups of seed ferns could have evolved into the carpels of flowers.

Earliest Flowers

Most paleobotanists assume that the first flowers were small, simple, and green in color and by modern standards were rather unattractive. Their petals and sepals were probably not clearly differentiated.

In November of 1998, Ge Sun and David Dilcher and their colleagues published their discovery of the oldest angiosperm fossil to date, estimated to be at least 122 million years old and possibly as old as 145million years. Either age qualifies it as the oldest.

The fossils were discovered in China, and the fruits show the characteristic enclosed ovule (a carpel) that is distinctive to angiosperms. It was given the scientific name Archaefructus liaoningensis. Given its great age, this find implies that angiosperms may have arisen as early as the Jurassic period, more than 145 million years ago.

Other early flowers produced pollen with a single aperture, or opening, a trait that the monocot branch of the angiosperms shares with cycads and ginkgos. Plant taxonomists believe that pollen with a single opening is an ancestral feature that some plants have kept as they evolved. The pollen of eudicots,with its three apertures, is thought to be a derived feature (that is, a later evolutionary development).

Recent studies of angiosperm evolution, using data from deoxyribonucleic acid (DNA) sequences, have led to the proposal that an obscure shrub from the South Pacific island of New Caledonia, called Amborella trichopoda, represents what is left of the ancestral sister group (a related organism that branched off before the evolution of another group of organisms) to all the angiosperms.

As a sister group to all the angiosperms, it is considered to be the most primitive (in an evolutionary sense) of the angiosperms and therefore should resemble what the ancestor to the angiosperms was like. It does possess some of the expected primitive traits for a primitive angiosperm, such as small, greenish-yellow flowers and a lack of vessels for conducting water from the ground to the leaves.

Angiosperm Classification

Approximately 97 percent of angiosperm species are classified as either Monocotyledones (monocots), with approximately 65,000 species, or Eudicotyledones (eudicots), with about 165,000 species. The monocots include such familiar plants as the grasses, lilies, irises, orchids, cattails, and palms. The more diverse eudicots include most of the familiar trees and shrubs (other than the conifers) and many of the herbaceous plants.

The remaining 3 percent of angiosperms are called the magnoliids, a group of plants considered to have primitive features, among them pollen with a single aperture. Many magnoliids also feature oil cells with ether-containing oils providing the characteristic scents of laurel and pepper, for example. The magnoliids are typically divided into the woody magnoliids and paleoherbs.

Woody magnoliids have large, often showy, bisexual flowers with multiple free parts. Magnolia trees and tulip trees (both in Magnoliaceae, or the magnolia family) are examples of this group. The paleoherbs have small, often unisexual flowers and usually just a few flower parts. Modern paleoherbs include the pepper family (Piperaceae), the birth-wort family (Aristolochiaceae), and the water lily family (Nymphaeaceae).

Recent studies of angiosperm evolution, using data from DNA sequences, have also sharpened the understanding of some of the relationships among monocots, eudicots, and magnoliids. If these groups are displayed as an evolutionary tree (or phylogenetic tree), the magnoliids are polyphyletic (that is, they do not share a single common ancestor).

The magnoliids branch off near the base of the tree on several different branches. The monocots are monophyletic (that is, they share a single common ancestor) and form a separate branch from among the magnoliid branches. The eudicots branch off last and represent the most diverse and evolutionarily complex group.

Geographic Origins

As hotly debated, perhaps, as exactly which group of plants were ancestral to the angiosperms is the question of where the angiosperms first evolved. Some botanists believe that angiosperms first developed in the Northern Hemisphere; others look at the Southern Hemisphere.

At the time angiosperms are proposed to have evolved, the continents were not arranged the way they are now. At that time, all of the world’s major landmasses were grouped into a supercontinent called Pangaea.

The southern part of this continent is referred to as Gondwana land, and the northern part is called Laurasia. Based on what is known about late Cretaceous angiosperms and their habitats, some scientists suggest that the westernmost, semiarid regions of Gondwana land may be the place where angiosperms first evolved.

As Pangaea broke up, the separate continents moved in different positions, resulting in new configurations. India collided with Asia, raising the Himalaya Mountains and the Tibetan Plateau. Antarctica slipped over the South Pole, and Australia became isolated. These plate movements created new climatic regimes, opening up new niches that were rapidly exploited by the angiosperms.

Diversification and Spread

Regardless of their geographic origins, by about ninety million years ago the flowering plants were well on their way to dominating the world’s flora. The early angiosperms were well adapted to drought and cold.

Adaptations that conferred resistance to these conditions included strong leaves, efficient water-conducting cells, and tough, resistant seed coats. Some woody flowering plants evolved the ability to lose their leaves, called the deciduous habit.

This characteristic allows the shutdown of metabolism during adverse environmental conditions, such as during seasonal droughts or winter weather. Because of greater climate instability during the past fifty million years or so compared to earlier times, the above-mentioned traits were important in allowing the flowering plants to adapt to different and often harsher climatic conditions.

Pollination

pollination by insects
pollination by insects

A major innovation that likely led to some of the great diversity seen in angiosperms was pollination by insects or other animals. As plants adapted to the various available pollinators, the pollinators also adapted to the plants, sometimes in very specific ways. Many pollination systems include specialized colors or markings, flower shapes, and flower scents.

This process of evolving "together" is called coevolution. Coevolution has also occurred between plants and their predators. Evolution of chemical compounds to deter herbivory have, in turn, led to adaptations in many animal groups to circumvent the toxicity of the chemical compounds.
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Angiosperm Life Cycle
Angiosperm Life Cycle

The word "angiosperm" comes from the Greek words for "vessel" and "seed" and translates roughly as "enclosed seed". In part, angiosperms (the flowering plants, phylumAnthophyta) are defined by the fact that their seeds are enclosed by an ovule. The life cycle of an angiospermis defined by the formation of the seed and its development to a full-grown plant which, in turn, produces seeds.

Angiosperms are vascular plants with flowers that produce seeds enclosed in an ovule—a fact that is recognized as the angiospermy condition.

Reproductive Flower Parts

In general, angiosperms have a floral axis with four floral parts, two of which are fertile. At the receptacle, or tip, of the axis there is an ovule-bearing leaf structure known as the carpel. The ovule or ovules can be found inside the pistil. Three portions compose the pistil: the ovary, the style, and the stigma, where the pollen usually germinates.


The mature ovule consists of the placenta, the integuments that are modified leaves that cover the entrance to the embryo sac, the micropyle, and the chalaza. These latter two parts of the ovule complement each other in their positions and functions.

While the micropyle receives and guides the pollen tube, the chalaza relates to the vascular supply of the ovule, nutrition, and support. The stamens, which are often composed of the filament and sporangia sacs that make up the anther, surround the pistil. Stamens carry the male gametes, and the pistil carries the female gamete needed for sexual reproduction.

It is believed that the great diversity and adaptability of the angiosperms is related to the presence of a unique reproductive cycle. This cycle consists of an alternation of generations and the production of a pair of spores on two types of sporophylls: microspores (which become male gametophytes) and megaspores (which become female gametophytes).

Male Gamete Development

The angiosperm reproductive cycle begins with the process of microsporogenesis, or microspore formation. The stamen consists of a filament and the anther, also known as the microsporangium. Inmost of the cases, the anther consists of four pollen sacs, or locules.

Within each locule, the archesporial cell develops through mitosis and extends as a row of cells throughout the entire length of the young anther. These mitotic cell divisions generate the anther wall, which is made up of several cell layers, the outermost of which transforms itself into the epidermis. The layer of cells belowthe epidermis is known as the endothecium.

During anther development, the endothecial cells acquire thickenings whose function is related to anther opening and pollen release. The innermost layer of the anther wall is the tapetum,whose primary function correlates with the nourishment of the young pollen and the deposition of the exine, a coating of the pollen grain.

As development proceeds, the sporogenous cells located below the tapetum transform into microsporocytes. These microsporocytes will undergo meiosis, and tetrads (units of four) of microspores will form.

Shortly after their formation, the tetrads separate into individual microspores. Each microspore is haploid, and often it will enlarge and separate from the tetrad, becoming sculptured by the deposition of sporopollenin and other substances that will turn into the ornamented surface of the pollen grain.

The second phase of pollen development is known asmicrogametogenesis. Themicrospore is the first cell of the gametophytic generation, the cell that generates themature pollen. The single-nucleus microspore develops into the male gametophyte before the pollen is released.

This developmental process occurs through two or three unequalmitotic divisions of the nucleus and subsequent cytokinesis (cell separation). The two daughter nuclei and cells differ in size and in form.

The larger cell represents the tube cell and nucleus,while the smaller cell represents the generative cell and nucleus. At maturity, the grain can be shred in two or three nucleate conditions. When the anther opens, the mature male gametophytes or pollen grains will be disseminated and ready for germination.

Female Gamete Development

The ovule (female sex organ) consists of two opposite ends: the micropyle, where the integuments come together, and a more distant end, where the ovular tissue is more massive. This part is also known as the chalaza, and it lies directly opposed to the micropyle.

The mature ovule is composed of three layers: the outer integument; the inner integument; and, underneath the integuments, the nucellus. During ovular development, one cell lying below the nucellar epidermis changes into a primary archesporial; this will divide to form the primary parietal cell and primary sporogenous cell.

The primary sporogenous cell functions as the megaspore mother cell, which divides meiotically, originating four haploid megaspores. In the majority of angiosperms, three of the megaspores will degenerate, and only the chalazal one will develop into the megagametophyte (embryo sac).

After the completion of the embryo sac stage, a series of cellular events occurs, ending with the formation of the mature embryo sac, ready for fertilization by the male gametes. The chalazal megaspore enlarges and undergoes threemitoses, giving rise to eight haploid cells. The mature megagametophyte consists of two groups of four cells located at both ends of the embryo sac.

The result is three antipodals at the chalazal end: the egg apparatus (consisting of the egg and two synergids at the micropylar end) and the polar nuclei. These two cells, present at both ends, usually fuse before pollination, and during fertilization they form the primary endosperm nucleus.

Pollination

The plant reproductive structures are now ready for the union of male and female gametes or fertilization, which eventually will produce a seed with a viable embryo and cotyledons. Before that step takes place, however, the pollen must be transferred from the anther to the stigma. Biotic agents (such as birds, insects, or mammals) or abiotic agents (such as wind or water) can accomplish this transfer process, known as pollination.

After landing on the stigma, pollen tubes will emerge through the grain apertures if the environ- ment is high in humidity. Successful germination of the pollen in the stigma requires nutrients. In most plants, growth of the pollen tube lasts between twelve and forty-eight hours, frompollen germination to fertilization.

Pollen germination starts with pollen-tube initiation, elongation, and penetration of the stigmatic tissue. During this period intense metabolic activity takes place, for the tube must synthesize membrane material and cell wall for growth and expansion. Simultaneously, at its tip the tube carries the vegetative cell nucleus, fol- lowed by the germinative cell.

Angiosperms have evolved complex breeding systems that ensure they will be pollinated by their own species. Today it is recognized that two pollination syndromes exist: self-pollination and cross-pollination. In self-breeding species, the pollen comes from the anther of the same flower.

In cross-pollination (or outcrossing) species, the pollen comes from the anthers of a different flower or even a different plant of the same species. In these plants, incompatibility in the stigma guarantees that only pollen from other flowers will germinate.

Fertilization

The union of one sperm with the egg is known as fertilization. However, several developmental processes in the vegetative and germinative cells prepare the two sperms for a process known as double fertilization. A mitotic division of the germinative cell generates the spermcells. This process that can take place on the growing pollen tube or inside the pollen grain.

In a growing pollen tube, the vegetative nucleus disintegrates and the sperm cells will take the lead and enter the embryo sac for successful fertilization. Usually, the interactions between the pollen grain and the pistil ensure that the sperm cells will often reach the micropyle of the ovule.

Once the spermreach themicropyle, the growth of other tubes stops. In the embryo sac (female gametophyte), four cells are located at themicropylar side.Of those four, the first pair that the spermcells will encounter are the synergids.

One of these is always bigger than the other and carries the filiform apparatus, a structure resembling hairs that degenerates after pollination and before fertilization. The synergids act as chemical attractants to the pollen tube, which penetrates the synergids via the filiform apparatus and then releases the two sperm cells.

One of the sperm cells will fuse with the egg, producing the zygote; the other sperm cell will fuse with the primary endosperm nucleus, generating the endosperm. The remaining cells of the female gametophyte are the antipodals; they usually degenerate after fertilization has taken place.

Seed and Fruit Formation

Once fertilization has occurred, the ovule will go through a series of metabolic steps ending with the formation of the seed and the fruit. The recently created zygote transforms into amulticellular and complex embryo that has two well-defined polar ends: the radicle, or primary root, and the embryonic apical meristem with the first leaves.

After successive mitosis, the mature endosperm usually grows close to the embryo and may provide nutrients needed for germination. The integuments will undergo further transformation, replication, and elongation and will become the seed coat—of variable texture, consistency, and colors, depending on the type of plant.

In general, after pollination or during fertilization, the ovary undergoes a series of physiological changes regulated by synchronized hormonal and genetic alterations that will modify the size of the parenchyma cells and its sugar and organic acids contents.

This process turns the ovary into fruit—in many cases familiar as the edible fruits familiar in human diets. The fruit provides nourishment for the seed until it ripens and drops to the ground, where the next stage in the life cycle begins.

Germination, Seedling Development, and Maturation

Seeds are released from the fruit in a large variety of ways that have evolved to ensure the survival of species. Whether ingested by mammals and passed through their feces to the ground, borne by wind on feathery "wings", or simply falling from rotting fruit that has abscissed and dropped from the plant, the seed must next undergo a process called germination, in which the embryo enclosed in the seed begins its growth. The embryo develops a hypocotyl (root axis) and a fleshy part known as the cotyledon; inmonocots there is one cotyledon, in dicots, two.

Germination requires certain conditions, such as the softening of the seed coat, moisture, and adequate warmth, to occur. During germination, the hypocotyl begins growing downward to become the root; the cotyledon(s) will develop into the shoot, stems, and leaves.

The process of germination results in the sprouting through the ground’s surface of the seedling, which will develop into the mature plant with flowers. The cycle then begins again.
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Angiosperm Plant Formation
Angiosperm Plant Formation
Angiosperms are flowering plants. Their formation entails development from embryo to seed, through germination to seedling, and finally to mature plant.

The life cycle of angiosperms (flowering plants) involves an alternation of generations between a dominant sporophytic (spore-producing) phase and a reduced gametophytic (gamete-producing) phase. The first cell of the sporophyte is the fertilized egg, or zygote, which undergoes repeated divisions, growth, and differentiation to form an embryo enclosed in the ovule.

After fertilization, the ovule is transformed into the seed, which germinates into a seedling. The seedling becomes the adult plant; the plant produces flowers inwhich the sperm and egg—representing, respectively, the male and female gametophytic generations—are formed. Fertilization occurs, and seeds are produced to continue the life cycle.


Dicot Embryo Formation

In most angiosperms, embryo development, or embryogenesis, is initiated with a division of the fertilized egg into a small apical cell and a large basal cell, forming a two-celled proembryo. The apical cell generates the embryo proper, and the basal cell forms a filamentous suspensor that anchors the embryo.

Two weeds, Capsella bursa-pastoris (shepherd’s purse) and Arabidopsis thaliana (mouse ear cress or wall cress), both belonging to the Brassicaceae family, have attained prominence as textbook examples of embryogenesis in typical dicots (plants with two cotyledons, or seed leaves; a monocot has one seed leaf).

In these plants, the apical cell of the proembryo divides by two successive longitudinal walls to forma quadrant that is immediately partitioned by transverse walls into an octant, composed of an upper and lower tier of four cells each.

The fates of the two tiers are already fixed in the octant embryo, as the upper tier forms the shoot apex and much of the cotyledons. The lower tier, in addition to providing derivatives to the remaining part of cotyledons, generates the hypocotyl, the radicle, and the root apex.

However, the central region of the root cap, known as the columella, and the quiescent center of the root apical meristem are derived from the terminal cell of the suspensor closest to the embryo, known as the hypophysis. The apicobasal pattern of the future seedling plant is established in the octant embryo.

Aseries of divisions separating eight peripheral cells from a core of eight inner cells heralds histogenesis in the embryo. The result is the formation of a sixteen-celled, globular embryo, in which the peripheral cells form the protoderm (precursor cells of the embryonic epidermis), and the inner cells differentiate into the procambium and ground meristem (precursors of the vascular tissues and ground tissues, respectively) of the mature embryo. This initiates the formation of radial-pattern elements made up of concentric tissue layers in the basal part of the embryo.

Dicot Embryo Formation
Dicot Embryo Formation
The globular stage of the embryo is completed by approximately three additional rounds of divisions, mostly in the inner core of cells. The suspens or attains its genetically permissible number of six to nine cells by this stage. Gradually the cells begin to lose connection from one another and from the embryo and disintegrate.

Emerging from the globular stage, the embryo expands laterally by cell divisions to formthe cotyledons and becomes heart-shaped. The heart-shaped stage is followed by the torpedo-shaped stage, in which elongation of the cotyledons and hypocotyl, as well as extension of the vascular tissues, occurs.

The basic body plan of a shoot-root axis becomes unmistakably clear at this stage, with the establishment of the shoot apical meristem in the depression between the cotyledons and the organization of a root apex by incorporation of derivatives of the hypophysis at the opposite end of the embryo.

During further growth, the cotyledons bend toward the hypocotyl (bent cotyledon or walking-stick shaped stage), and the embryo is phased into the mature stage. A mature embryo of Arabidopsis has fifteen thousand to twenty thousand cells and, under favorable conditions of growth, develops in about nine days fromthe time of fertilization to the mature embryo stage.

Sensitive genetic screens have led to the isolation of Arabidopsis mutants defective in apicobasal and radial patterning of embryos. Characterization of the mutant genes and their protein products has unraveled to some extent the molecular components of the embryonic pattern-forming system in this plant.

Monocot Embryo Formation

The early divisions of the zygote in monocots follow the same pattern as in dicots. However, in the Poaceae (grasses) family, which includes wheat and the other cereals, the sequence and orientation of later divisions in the proembryo are irregular, resulting in highly complex mature embryos. The main feature of the cereal embryo is the development of an absorptive organ known as the scutellum (considered equivalent to the single cotyledon).

Other organs of the embryo for which there are no counterparts in the dicot embryo are a sheath like tissue covering the root (coleorhiza), a tissue that covers the shoot (coleoptile), and an internode known as the mesocotyl. On one side of the coleorhiza there is also a small, flaplike out growth called the epiblast.

Embryo Maturation to Seed

Embryo Maturation to Seed
Embryo Maturation to Seed
As the embryo matures, the ovule progressively desiccates to become the seed enclosed within the ovary. Concomitantly, the integuments of the ovule harden to form the protective seed coat. Within the ovule itself, the primary endosperm nucleus formed after double fertilization begins to divide, ahead of the zygote, to produce the endosperm charged with nutrient substances. In seeds ofmany plants, including Arabidopsis, Capsella, bean, and pea, the endosperm is utilized by the developing embryo.

In other plants, especially the cereals, the bulk of the seed (grain) is made up of the endosperm surrounding the small embryo. The mature embryo enclosed in the seed consists of an axis bearing the radicle (embryonic root) at one end and the plumule (the embryonic shoot consisting of the shoot apex and one or two leaves) at the other end, and one (in monocots) or two (in dicots) cotyledons.

The part of the embryo axis above the point of attachment of the cotyledon(s) is known as the epicotyl, whereas the part below the attachment point connecting to the radicle is called the hypocotyl.

Seed Germination

The dry seed enclosing the mature embryo may not germinate immediately; if it does not, it enters a period of quiescence or dormancy. Quiescent seeds germinate when provided with the appropriate conditions for growth, such as water, a favorable temperature, and the normal composition of the atmosphere.

Dormant seeds germinate only when some additional hormonal, environmental, metabolic, or physical conditions are met. In almost all seeds, the first part of the embryo to emerge during germination is the radicle. It forces it way through the soil and forms the primary root of the seedling. However, the manner in which the shoot emerges and develops varies considerably in different seeds.

In the epigeous type of germination (for example, in beans), emergence of the radicle is followed by the elongation of the hypocotyl, which arches above the soil surface as a hook. As the hook straightens, it pulls out the cotyledons and plumule above the soil surface. In the hypogeous type of germination (in peas, for example) the cotyledons enclosed within the seed coat remain in the soil during germination.

It is the epicotyl that arches above the soil surface, and as the hook straightens out, it carries the plumule along with it to the surface of the soil. In the monocot, such as the onion, after emergence of the radicle the single cotyledon arches above-ground and subsequently straightens.

Members of the Poaceae display a type of germination in which, following the outgrowth of the radicle, the coleoptile enclosing the plumule grows out of the grain and appears above the soil. The seedling leaves force theirway, breaking the coleoptile, and appear outside as the first photosynthetic organs.

The growth of the coleoptile during germination of grains is facilitated by the elongation of the mesocotyl. These various types of germination ensure an efficient use of food materials stored in the embryo or in the endosperm for the growth of the seedling until it becomes autotrophic.

Embryo to Adult Plant

Although the question as to whether the seedling will become a gigantic tree or a small, herbaceous plant is determined by its genetic blueprint, certain common postgermination growth and developmental episodesmark the development of the seedling into an adult plant.

In dicots, continued growth of the primary root produces an extensively branched root system consisting of secondary roots or lateral roots. In monocots, the primary root disintegrates shortly after it is formed, and so the root system is constituted of numerous adventitious roots which arise from the base of the stem.

Although the cotyledons retain their photosynthetic capacity for some time after germination of the seed, the seedling becomes completely autotrophic as the shoot apex produces new leaves and branches arise in the axils of leaves.

These activities are coordinated by the division of cells in the root and shoot apical meristems and the differentiation of cells into specialized tissues and organs. The shoot and root apical meristems, considered analogous to the stem cells of animals, remain active throughout the life of the plant and, hence, are known as indeterminate meristems.
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8:37 PM
Angiosperms - Chinese Bladdernut Buds
Angiosperms - Chinese Bladdernut Buds

The name "angiosperms" has long been used by botanists to refer to the flowering plants, a group of approximately 235,000 species. All angiosperms are members of the phylum Anthophyta.

The name "angiosperm" is actually derived from two Greekwords, angeion,meaning "vessel" or "container", and sperma, meaning "seed". The name was given in reference to the fact that the seeds of all flowering plants develop from ovules that are enclosed in a structure called a carpel.

This characteristic sets the angiosperms apart from all other plants, which either do not have seeds or have seeds that are not developed in structures resembling a carpel. Although the name angiosperm is used widely, plant taxonomists and many botanists typically refer to them by the more formal name Anthophyta, the phylum that contains the flowering plants.

Unique Features of Angiosperms

In addition to possessing enclosed seeds, Anthophyta differs from other plant phyla in a number of ways. The most obvious distinguishing feature is the flower, a complex structure containing the reproductive parts of the plant. The reproductive structures in other plants are much less complex and showy. The angiosperm life cycle differs from that of almost all other plants.


The sporophyte is the dominant, diploid stage and is the more visible form of the plant, with the leaves, stems, roots, and flowers. The haploid gametophyte is confined to life inside the ovary or anther of the flower, unlike the typically free-living gametophytes of most other plants.

Fertilization is also unique in angiosperms. Many angiosperms rely on insects or other animals to transfer pollen from one flower to another. Pollen grains produce two haploid sperm that travel through a pollen tube from the stigma into the ovary of the flower and into one of the embryo sacs.

Within the embryo sac one of the sperm fertilizes the egg, which will lead to formation of the diploid embryo, and the other sperm fuses with two or more polar nuclei to form the endosperm, which will nourish the embryo and young seedling. This process is often referred to as double fertilization. Other, less obvious features set Anthophyta apart as well, including a unique vascular anatomy, pollen structure, and various biochemical characteristics.

Size and Geographic Diversity

There are approximately 235,000 species of flowering plants, and they are found in almost all terrestrial habitats, with the exception of extremely high elevations and some polar regions. As mall proportion of flowering plants are aquatic (that is, found in freshwater habitats), and an even smaller number aremarine (found in saltwater habitats).

The greatest species richness is in tropical regions, especially tropical rain forests, and species richness steadily decreases at increasing latitudes north and south of the equator.

Angiosperms have been so successful in terres- trial ecosystems that they represent the majority of the herbs and shrubs and many of the trees as well. The diversity of growth forms is tremendous, represented by such diverse families as Poaceae (grasses and bamboos), which have greatly reduced and modified flowers; Cactaceae (cactuses), which have spines instead of leaves and very showy flowers; and Lemnaceae (duckweed), which has a highly reduced plant body sometimes comprising a single leaf with no true roots or stem and the smallest flowers of any angiosperm.

Other families include Asteraceae (sunflower or aster family), with reduced disc and ray flowers crowded together into inflorescences called heads; Salicaceae (willow family), a widespread, water-loving family of trees and shrubs with reduced flowers arranged in catkins; and Orchidaceae (orchid family), with some of the showiest and most intricate flowers of all, which have extremely numerous and minute seeds.

Economic Importance

Economically, angiosperms have made a profound impact. Essentially all of the world’s food crops, from rice, wheat, and corn to other fruits and vegetables, are derived from flowering plants. In fact, it is almost impossible to think of more than a handful of foods or food ingredients from plants that are not flowering plants.

Angiosperms - Stachyurus
Angiosperms - Stachyurus

The same is true of ornamental plants. Although a few gymnosperms (such as conifers) and ferns are common as ornamentals, most of the remaining plants, even many valued for their foliage rather than their blooms, are flowering plants.

The only area where angiosperms do not dominate economically is in forest products, where conifers account for a significantly larger proportion of the harvest, but even there, hardwoods predominate for certain applications.

Medicine has also reaped many benefits from angiosperms. In fact, it was primarily the herbalists, from the Middle Ages to the Scientific Revolution, who expanded humankind’s understanding of flowering plants. Knowledge of flowering plants for food and medicine among many indigenous peoples has always been wide spread.

Modern medicine has capitalized onmuch of this knowledge and has even expanded the search for new medicines. Flowering plants have been the original source of many precursors to modern medicines, including aspirin (willows, Salix), quinine (Cinchona species), and digitalin and digoxin (Digitalis species).

Lifestyle Diversity

Along with the diversity in structure comes a diversity in lifestyles. Most angiosperms are free-living, that is, receiving their primary energy and carbon from photosynthesis and their nutrients from the soil.

A few groups of plants receive their energy or nutrients in other ways. Some are saprophytes, which receive their energy and carbon from decaying organic material in the soil and their nutrients from other soil components, much like other plants.

Some of the best-known saprophytes are in Ericaceae (heath family). Their most distinctive feature is that they are either white or some shade of pink or red and are never green. Monotropa uniflora (Indian pipes), for example, is a ghostly white and has no chlorophyll.

Parasitism is an alternative for some angiosperms. One well-known parasite is the mistletoe (Loranthaceae), popular as a Christmas decoration, which is a branch parasite on trees. Many types of mistletoe have green foliage and therefore receive some of their energy from photosynthesis, but their primary nourishment comes from the host tree.

Some species have foliage that is brown or yellow and do not photosynthesize much at all. The seeds of mistletoe are spread from tree to tree when birds eat their berries and defecate the seeds on the branch of another tree. Probably the most unusual parasite is Rafflesia, from Malaysia and Sumatra.

It parasitizes species of Tetrastigma, a vine that grows on the forest floor and has no stems or leaves of its own. When it blooms it has the largest flowers in the world, and it is often called the corpse flower because it has a very strong odor, like that of rotting flesh.

Other parasites receive varying proportions of their energy and nutrients from their host and conventional means, and when the contributions are nearly equal they are referred to as hemiparasites. Hemiparasites are common in Castilleja (paint-brushes), and many species invade the roots of other plants to obtain part of their nutritional needs.

Aunique approach to obtaining nutrients is represented by insectivorous plants, commonly known as carnivorous plants These plants use a variety of adaptations for trapping and absorbing nutrients from insects.

Sundews (Droseraceae) have special glands on their leaves that excrete a sticky fluid that traps insects like flypaper. Pitcher plants (Nepenthaceae and Sarraceniaceae) have special tubular leaves that resemble cups or pitchers.

The inside of the leaves fill with water near the base, and the lip and inside surface of the pitcher are slippery. Once an insect gets inside, it slips into the water at the bottom. Venus’s flytrap (Dionaea, also in Droseraceae) is even more intricate, with leaves specially modified with traps that spring shut when an insect lands or walks on them.

There is even an aquatic carnivore, the bladderwort (Utricularia), which has saclike leaves with small openings that can close after a small aquatic insect or crustacean is sucked in.Although insectivorous plants do obtain some of their nutrients from insects, they also obtain nutrients from the soil or, in the case of bladderworts, surrounding water.

Angiosperm Classification

Traditionally Anthophyta has either been considered as a single class Angiospermae or Magnoliopsida, with two subclasses, or has been divided into two classes, Eudicotyledones, or Magnoliopsida, and Monocotyledones, or Liliopsida. The second of these two options is more commonly accepted by contemporary plant taxonomists, and the two classes are often referred to by the common names dicotyledons or dicots and monocotyledons or monocots, respectively.

The monocot/dicot dichotomy has long been considered a major evolutionary split in the angiosperms. The two classes a differ fromeach other in a number of ways. Monocots generally have bladelike leaves with parallel venation, whereas dicots more typically have pinnate or palmate venation. Monocots have fibrous root systems without taproots; dicots typically have taproots.

The flower parts in monocots occur typically in threes, whereas they occur most often in fours and fives in dicots. Monocots lack cambial secondary growth,which is common in dicots. Monocots have scattered vascular bundles in their stems, as opposed to the more orderly arrangement seen in dicot stems.

It has long been proposed that the monocots branched off fromthe dicots very early in the evolution of the angiosperms, but until recently it was difficult to sort out the probable events and the resulting classification system that would be needed to reflect them.

With the advent of molecular tools, such as deoxyribonucleic acid (DNA) sequencing, the study of early angiosperm evolution is getting much more attention. It has now become clear that, if the classification system is to reflect evolutionary history, Anthophyta must be divided intomore than just two classes.

Currently there is no agreement on how many other classes there should be, but Monocotyledones and Eudicotyledones will retain most of the taxa. This new approach to the classification of Anthophyta has also resulted in changing the common name of the "dicots" to "eudicots", meaning "true dicots".

Many of the remaining taxa not included in the monocots or eudicots are now often referred to as magnoliids and are considered to represent taxonomic groups that have branched off fromthe early angiosperms before the monocot/eudicot split. Some of these groups include the orders Magnoliales (which includes Magnoliaceae, long considered as having many primitive characteristics), Winterales, and Laurales.

The placement of a few taxa, such as Ceratophyllaceae and Chloranthaceae,is particularly controversial. With continued analyses of DNA sequences it is hoped that a clearer picture of the relationships among the magnoliids and related taxa will be obtained and a more phylogenetically based classification system can be devised.
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9:27 PM
Biochemical Coevolution in Angiosperms
Biochemical Coevolution in Angiosperms
Flowering plants, or angiosperms, produce many compounds that are not directly related to growth and development. These secondary metabolites arise from primary metabolic pathways and act as antiherbivory mechanisms, allelochemicals, or attractants.

Secondary metabolites are biochemicals produced by plants in response to selection pressures. These pressures may be from herbivory, competition, or the need for pollination.

As plants produce compounds to enhance their survival, predators, competitors, and pollinators react and evolve means of adjusting to the plant’s efforts. Chemically simple secondary metabolites may be widespread throughout angiosperm (flowering plant) families, whereas more complex chemicals are often restricted to a single species.

Secondary metabolites are often under high selection pressures, causing individual compounds to have very limited distributions and making them useful in determining the evolutionary relationships between taxonomic groups. Presence of secondary compounds influences the activities of organisms interacting with the plants and, over long periods of time, influences evolution of those species.

Antiherbivory Mechanisms

Antiherbivory chemicals may have a wide range of effects on herbivores (plant-eating animals). Many compounds merely deter grazing. Crystals produced from calcium oxalate (raphides) may be ejected from the vacuoles of cells along with proteinaceous toxins, causing tissue swelling in the mouth of an offending herbivore.

Many monocotyledonous plant families, such as Liliaceae, Heliconiaceae, Rubiaceae, and Arecaceae, produce this type of antiforaging device. This type of defense is especially notable in young tissues that have not developed the toughness found inmature leaves as a herbivory deterrent. Red oaks produce tannins in response to gypsy moth attacks, reducing further herbivory.

Continued feeding on plants containing tannins would lead to slow starvation of a herbivore, as its digestive system could not absorb proteins. The same tannins are not deterrents to squirrels. Squirrels harvest acorns and bury them for later consumption, providing a food source for the squirrel and a dispersal mechanism for the acorn.

Beavers provide another example of the interaction between plant and animal evolution. Some of the beaver’s preferred foods include species that are unpalatable or toxic to other mammals, such as bracken fern, nettles, thistles, and skunk cabbage. This gives the beaver a largely uncontested food source that may involve a metabolic "cost" to the animal.

Other antiherbivory chemicals result in effects more severe than mere deterrence of feeding. Alkaloids such as caffeine, nicotine, and strychnine are potent anti herbivory mechanisms, causing convulsions, comas, and even death in herbivores. These effects may not occur in all herbivores.

Strychnine, for example, is produced by the fruit of some plants that may be eaten by birds without ill effects, but in mammals the same fruit causes failure of the central nervous system and induces seizures.The plant reduces herbivory by mammals, and the seeds get dispersed by birds that are able to detoxify the strychnine.

Grains and seed crops, such as wheat and peanuts, which are particularly attractive to animal and insect herbivores, often produce cyanogenic glycosides that release hydrogen cyanide as the tissues are digested. This compound inhibits cellular respiration, thus killing the herbivore. In each case members of a plant population are consumed by the herbivore, but future generations are spared by the loss.

Allelochemicals

Allelochemicals
Allelochemicals
Allelochemicals are compounds produced by an organism that interfere with the growth or development of another organism.Many phenolic acids act as allelo chemicals, inhibiting root growth of competing species. Many grains are known to release ferulic acid and caffeic acid into the soil, thus inhibiting the germination of weed species.

Phenolics may also act as antifungal compounds, increasing in concentration with fungal infection, thus protecting the plant from further attack. Phenolic compounds produced in tobacco and tomato leaves reduce the growth of these plants’ natural predator, tobacco hornworm, without affecting the growth or activity of the hornworm’s natural predator.

Allelochemicals produced in response to injury by herbivores may also attract predators of the herbivore.Wastes from many species of caterpillars induce the release of terpenoids from green leaves that attract parasitoid insects.

Production of specific combinations of volatiles on the part of the plant signals the predator, which will then reduce further herbivory. The plants have evolved the signal in response to herbivory, and the predators have evolved the ability to detect the signal indicating the location of their host.

Lectins are widely distributed carbohydrate-binding proteins, most commonly found in the Leguminoseae (legume) family. When found in the seeds, these compounds act as broad-spectrum insecticides, whereas in the roots of legumes they maintain bacterial relationships in nitrogen fixing nodules, providing the plant with a source of nitrogen unavailable to plants not producing nodules.

Attractants

Attractants
Attractants
Terpenoids and aliphatic compounds are often the components of essential oils of plants. The volatile nature of these compounds produces a distinctive odor that attracts pollinators. Composition of the volatile compounds often closely matches the natural pheromones produced by the pollinator, mimicking the chemical scent of a female insect in an attempt to attract male pollinators.

Pheromone mimicry is found primarily in members of the Orchidaceae, which are often dependent on single species of wasp for pollination. Other plants may mimic the odor of food. The smell of rotting flesh, attractive to flies, is produced via ammonia and alkylamines, such as cadaverine and putrescine. Methylesters may attract moth pollinators by mimicking the sweet smell of fruit.

Flavonoids often provide color to fruits and flowers and act as visual cues for pollination. Reds, blues, or yellows in varying patterns stand out against a background of green leaves, helping pollinators locate the flower.

Species may have minor chemical differences in their flavonoids that allow for the determination of identity, hybridization between species, and possible coevolution with pollinators. For example, tropical flowers tend to have a more intense red color from anthocyanins than do temperate flowers.

This difference correlates with differences in pollinator preferences, indicating a role by natural selection. Birds, such as hummingbirds, prefer red to yellow, whereas bees are not able to discern reds but are attracted to yellows. Carotenoids, such as xanthophyll and beta-carotene, give fruits and flowers distinctive yellow and orange colors.

Color patterns are also important in attracting pollinators. Butterflies are attracted to red/yellow color patterns. Flavonoid compounds not only impart color but also may modify color patterns by absorbing ultraviolet (UV) light.

Bees are capable of seeing in the UV range, so the presence of flavonoids may alter the bees’ perception of the flower. The patterns may also create cues as to the location of nectaries within the flower, guiding the pollinator to its reward.
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3:21 AM
Bromeliaceae
Bromeliaceae
The family Bromeliaceae comprises a group of perennial, monocotyledon herbs or trees that often age slowly.

Important ornamentals (called bromeliads) as well as sources of food and medicines, Bromeliaceae have substantial economic value and are widely cultivated. The colors of the leaves offer decorative foliage, and the flowers are of astonishing hues due to the rich content of pigment-forming substances known as anthocyanins.

Based on ovary position, habit, and floral and pollen morphology, the family Bromeliaceae has been split into three subfamilies: subfamily Pitcairnioideae, subfamily Tillandsioideae, and subfamily Bromelioideae.

There are fifty-six genera and approximately twenty-six hundred species, growing mostly in the neotropical regions of the world, from Virginia to southern Argentina. One species, Pitcairnia feliciana, originated in Africa. This interesting family can nevertheless occupy a variety of ecologically diverse environments, ranging from the dry deserts in Peru to the highest montane forest in the Andes Mountains.


Appearance and Structure

The Bromeliaceae family shares a basic ground plan of construction that consists of branches (ramets) and an inflorescence that follows a repetitive pattern when growing. However, modifications, in the form of reductions, of this basic plan have evolved in different subfamilies.

The basic pattern consists of sympodial branching, a rhythmic type of growth in which the axis is built up by a linear series of shoot units, each distal unit developing from an axillary bud located on the previous shoot unit. This pattern of development leads to a series of condensed ramets with terminal flowers. Roots, when present, usually emerge from the lower half of each ramet.

Growing Habit

Growing Habit
Growing Habit
Bromeliaceae range from small plants, such as some miniature Tillandsia, to very tall individuals, such as Puya raimomndii, reaching up to 32 feet (10 meters) in height. They can be epiphytes, that is, plants that use other species as support without harming them, or terrestrial. Some grow on top of rocks, and some are carnivorous.

Those specieswhose leaves are born from a common place in the stem (in a rosulate shape) can develop the tank form, also known as phytotelma, that is common in genera such as Aechmea and Brocchinia.

These phytotelma harbor a variety of insects and small vertebrates that grow in small pools of water and old leaves that collect at the bottom of the “tank.” The tanks accumulate water and partially dissolved organic matter, creating a nutrient rich substrate as a continuous supply of moisture. Other Bromeliaceae do not form tanks; instead, they have fully functional roots and specialized hairs for water absorption.

Scales

Physiological adaptations to different environments among some species correlate with the presence of a highly evolved type of foliar hair (or trichome) known as a scale. The scales may cover the entire surface of the leaf, sometimes appearing in different locations and patterns; they absorb atmospheric water through capillary action, like blotting paper, and the water is later transported to the leaf tissue, where it is stored in the parenchyma.

Division of the scale in two parts—known as the shield, or trichome covering, and the water absorption cells—is what makes Bromeliaceae unique. When water is scarce the scale shrinks, and when water is present the shield cells expand. Scales protect the leaves against transpiration and reduce water evaporation during the dry periods.

Flowers and Pollination

Flowers and Pollination
Flowers and Pollination
The flowers of Bromeliaceae are generally hermaphroditic (functionally unisexual). Their shape can be radial or slightly asymmetric, and the number of floral parts known as sepals and petals is always three. The stamen arrangement is in two whorls, with three stamens in each one. The ovary can be superior or inferior, and the placentation (position of the ovules) is mostly axial.

Septal nectaries are always present at the base of the flower. The sepals are distinguished fromthe petals by their color and size. The petals show bright colors, while the sepals may remain mostly in green hues. Fruits are usually a capsule or a berry, and the seeds are winged.

The bloom of Bromeliaceae flowers is usually odorless, although some species may have scented flowers, indicating pollination by nocturnal moths or butterflies.However, their abundant secretion of nectar indicates that the plants are pollinated primarily by birds.

Uses

The main uses of Bromeliaceae are as textile fiber, food, medicine, and ornamental plantings. In the food category the pineapple, Ananas comosus, is the most widely used species. The medicinal properties of pineapple are based on the presence of bromelain, a proteolitic (protein-breaking) enzyme that is widely used to treat inflammation and pain.

Serotonin, a neurotransmitter, is also present, and steroids from the leaves possess estrogenic activity. Thirteen species of Bromeliaceae are used as a source of textile fibers; for example, hammocks are made from the fibers of Aechmea bracteata and of pineapple.
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10:42 PM
Organ Pipe Cactus NM (AZ)... I've been there; I camped there with my wife; it was exhilarating! Sadly, the threat from illegal boarder crossings and drug trafficking will forever prohibit my return.
Wild cacti

Succulents are fleshy plants that store water in natural reservoirs such as stems or leaves. Cacti are a group of flowering plants; all cacti are succulents.

The Cactaceae family includes about 1,650 to 3,500 species of cacti and succulents classified in 130 genera. Because they live in harsh, arid environments, these fleshy, spiny perennial plants have developed a variety of unique characteristics for protection and to retain water, reduce evaporation, and resist heat.

Cacti

The word “succulent” is derived from the Latin term sucus, meaning sap. All cacti are succulents. The word “cactus” is derived from the Greek term kaktos, describing thistles. Botanists estimate cacti first existed during the Mesozoic era, about 130 million years ago. Limited cacti fossil evidence exists (the earliest known specimen is about forty thousand years old).


Cacti vary in size. The Copiapoa laui is a spherical plant several millimeters in diameter, while the Pachycereus weberi is cylindrical, stands more than 20 meters tall, and can weigh more than 25 tons. Cacti often develop bizarre shapes to cope with arid conditions. Some stems are flat, and others are puffy. Many consist of jointed segments, while others have one round stem.

Cacti stems swell when storing water. Surface ridges and grooves gather water. Roots extend in a wide area near the soil surface, to capture any moisture. The pincushion, barrel, saguaro, prickly pear, night-blooming cereus, and Christmas cactus are some of the most familiar cacti.

Unlike other plants, cacti have areolas on stems where branches, spines, glochids (bristles), leaves, and flowers grow. Spines protect and shade the plant and its seedlings from predators and ultraviolet radiation and serve as condensation sites. Known as crassulacean acid metabolism (CAM), photosynthesis in cacti is reversed from the process in other plants. Stems have chlorophyll because leaves are either absent or tiny.

In order to exist in a reality of unwavering love, honor and compassion, you must create that in the here and now. --Sandra Walker
Blooming cacti

At night, instead of day, cacti open the stomata on their stems to collect carbon dioxide and expel oxygen. The carbon dioxide is stored as organic acids for conversion to sugar during the day. Because the temperature is cooler when the stomata are opened, less water is lost. During the day, the closed stomata prevent evaporation from occurring.

Life Cycles of Cacti

Cacti grow slowly and can live more than a century. Flowers usually bloom in late spring and vary in color, size, and shape. Seeds are inside the fruits that blossoms produce. Some cacti grow from seeds if they are shaded and not consumed by predators.

Other cacti emerge from stems and take root where they fall. Artificially, cacti can also take root from cuttings. Diverse insects and animals are attracted to the flowers and assist in their pollination. Some birds nest in holes in cacti stems.

Pink Cactus Flower..... Maybe I will go back to my El Paso roots, it won't require as much attention or water!
Pink Cactus Flower

Distribution of Cacti

Cacti grow in deserts, prairies, mountains, and tropical climates and have developed a tolerance for extreme conditions. Cacti are indigenous to North, Central, and South America. The Epiphyllum species live in tropical trees. Other cacti grow in rocky places.

Some Chilean cacti in the Atacama Desert secure water from sea fog. The largest and most diverse population of cacti is in Mexico. The prickly pear is the most widely distributed cactus, ranging from near the Arctic circle to southern South America.

Uses of Cacti

Cactus fruits are edible by humans and animals and used as livestock forage, as a water source, for fuel, and to erect organic barriers. Spines are used as needles and fishhooks, and fibers are twisted into rope. Historically, peyote is a ceremonial hallucinogenic, and other cacti have medicinal purposes. While no cacti are poisonous, some species have unpleasant chemicals that discourage predators.

Some hybrids have naturally occurred, and cactus segments can be detached at joints to graft to artificially unique plants. Because of poaching, cacti are considered endangered plants, with some species threatened by extinction, and are federally protected at Saguaro National Park (in Arizona’s Tucson Basin) and Organ Pipe Cactus National Monument (in Arizona’s Sonoran Desert).

Succulents

Although they share many traits with their close relatives the cacti, other succulents do not have areolas. Succulents vary in shape and size. Some are as tiny as peas, while others are large as livestock. Succulents take many forms, including that of the string of beads (Senecio rowleyanus). Yucca and jade plants are two of the most familiar succulents.

Colorful succulents
Colorful succulents

Because of evolutionary adaptation to endure climatic extremes, succulents have small leaves and spongy tissues that keep water for prolonged durations. Succulents retain water to with stand such environmental stresses as drought, scorching wind, shallow or salty topsoil, steep locations, and over-crowding by other plants.

Succulents keep flower stalks and fruit until all the water is depleted from them. They have a thick skin, which is waxy, and sometimes alter their shape while adjusting to differences of light and moisture. Most succulents are gray, although a few are colored lilac, pink, light green, beige, or ivory, often in patterns that may serve as camouflage.

The greatest quantity and most diverse succulents can be found in Mexico and South Africa, which have thousands of species. New species are still being discovered because of variations arising from environmentally triggered adaptations. Some succulents, such as the Argyroderma, are abundant, growing in thick clumps.

The rarer succulents include Conophytum burgeri, which lives on only one South African hill. Succulents are threatened by overgrazing and industrial and agricultural development of habitats. Some succulents, particularly aloe, have healing juices to soothe burns.

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