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4:34 AM
Botany
Botany
Botany is a very old branch of science that began with early people’s interest in the plants around them. Plant science now extends from that interest to cutting-edge biotechnology. Any topic dealing with plants, from the level of their cellular biology to the level of their economic production, is considered part of the field of botany.

History and Subdisciplines

The origins of this branch of biology are rooted in human beings’ attempts to improve their lot by raising better food crops around 5000 b.c.e. This practical effort developed into intellectual curiosity about plants in general, and the science of botany was born.

Some of the earliest botanical records are included with the writings of Greek philosophers, who were often physicians and who used plant materials as curative agents. In the second century b.c.e. Aristotle had a botanical garden and an associated library.

As more details became known about plants and their functions, particularly after the discovery of the microscope, a number of subdisciplines arose. Plant anatomy is concerned chiefly with the internal structure of plants. Plant physiology delves into the living functions of plants. Plant taxonomy has as its interest the discovery and systematic classification of plants.



Plant geography, also known as geobotany or phytogeography, deals with the global distribution of plants. Plant ecology studies the interactions between plants and their surroundings. Plant morphology studies the form and structure of plants. Plant genetics attempts to understand and work with the way that plant traits are inherited.

Plant cytology, often called cell biology, is the science of cell structure and function. Economic botany, which traces its interest back to the origins of botany, studies those plants that play important economic roles. These include major crops such as wheat, rice, corn, and cotton.


Ethnobotany is a rapidly developing subarea in which scientists communicate with indigenous peoples to explore the knowledge that exists as a part of their folk medicine. Several new drugs and the promise of others have developed from this search.

At the forefront of botany today is the field of genetic engineering, including the cloning of organisms. New or better crops have long been developed by the technique of crossbreeding, but genetic engineering offers a much more direct course.

Using its techniques, scientists can introduce a gene carrying a desirable trait directly from one organism to another. In this way scientists hope to protect crops from frost damage, to inhibit the growth of weeds, to provide insect repulsion as a part of the plant’s own system, and to increase the yield of food and fiber crops.

The role that plants play in the energy system of the earth (and may someday play in space stations or other closed systems) is also a major area of study. Plants, through photosynthesis, convert sunlight into other useful forms of energy upon which humans have become dependent.

During the same process carbon dioxide is removed from the air, and oxygen is delivered. Optimization of this process and discovering new applications for it are goals for botanists.
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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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10:14 PM
Seaweeds that are brown to olive-green in color belong to the phylum Phaeophyta, or brown algae, which includes between fifteen hundred and two thousand species.

Brown algae (phylum Phaeophyta) are familiar to most people as brown or dark green seaweeds. Some brown algae are microscopic in size, but many are relatively large: One giant kelp measured 710 feet in length. All brown algae are multicellular.

Appearance and Distribution

Brown algae have a body, called a thallus, which is a fairly simple, undifferentiated structure. Some thalli consist of simple branched filaments. Some brown algae have more complex structures called pseudoparenchyma because they superficially resemble the more complex tissues of higher plants.

Giant kelp have a thallus that is differentiated into a holdfast, a stipe, and one or more flattened, leaflike blades. The holdfast functions as the name implies, and holds the rest of the organism to the substrate.


It is a tough, sinewy structure resembling a mass of intertwined roots. The stalk that constitutes the stipe is often hollow, with a meristem (a zone of growing tissue) either at its base or at the blade junctions. Because the meristem produces new tissue at the base, the oldest parts of the blades are at the tips.

The blades, which, like most of the rest of the giant kelp body, are photosynthetic, may have gasfilled floats called bladders toward their bases, which may contain carbon monoxide gas. The function of this particular gas has not yet been determined.

The vast majority of species are marine, living in cold, shallow ocean waters, and may be the dominant plant life on rocky coastlines. The giant kelp can be found in waters around 100 feet deep. Only 4 of the 260 identified genera occur in fresh water. Brown algae of the order Fucales are commonly called rockweeds; kelp belong to the order Laminariales.

Brown algae are less common in tropical and subtropical areas. However, in the Caribbean region, sargassum (large masses of brown algae having a branching thallus with lateral outgrowths differentiated into leafy segments, air bladders, or spore-bearing structures) make up large floating mats; they gave their name to the Sargasso Sea.

Pigments and Food Reserves

The color of the brown algae can vary from light yellow-brown to almost black. Its color reflects the presence of varying amounts of the brown xanthophyll pigment fucoxanthin, a carotenoid pigment, in addition to chlorophylls a and c. The main food reserve is a carbohydrate called laminarin, although giant kelp can also translocate mannitol. Algin (alginic acid) can be found in or on the cell walls and may comprise as much as 40 percent of the dry weight of some kelps.

Reproduction

Reproductive cells of brown algae are unusual in that their two flagella are located laterally, instead of at the ends. The only motile cells in the brown algae are the gametes or reproductive cells. In the common genus Fucus, separate male and female thalli are produced. Fertile areas called receptacles develop at the tips of the lobes of the thallus. Each receptacle has pores on the surface.

These pores open into special spherical, hollow chambers called conceptacles, in which the gametes are formed. Eight eggs are produced in the female structure, while sixty-four sperm cells are produced in the male structure. Eventually, both eggs and sperm are released into the water, where fertilization takes place and the resulting zygotes develop into mature thalli.

Economic Uses

Brown algae have several uses and applications for humans. Giant kelp is eaten, and one species found in the Pacific Ocean has been used, in chopped-up form, as a poultice applied to cuts.

Algin, a colloidal substance produced by brown algae, is used as a thickener or stabilizer in commercially produced ice cream, salad dressing, beer, jelly beans, latex paint, penicillin suspensions, paper, textiles, toothpastes, and floor polish. Brown algae, with its high concentration of the element iodine, has been used to treat goiter, an iodine-deficiency disease. Kelp, also high in nitrogen and potassium, has been used as fertilizer and as livestock feed.

Some types of brown algae, such as Fucus, contain either phenols or terpenes. Botanists believe these substances may discourage herbivory. These substances also have been shown to possess microbe- and cancer-fighting properties. Brown algae is the subject of continuing research in these areas of medicine.
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3:09 AM
Bryophytes
Bryophytes
Bryophytes comprise three phyla of nonvascular plants, which generally lack the specialized conductive tissues (xylem and phloem) that are found in the vascular plants, are small in size, and are distributed worldwide in moist, shady habitats.

Bryophytes (from the Greek word bryon, meaning “moss”) were once grouped together into one large phylum. Many botanists today recognize that these organisms belong to at least three distinct phyla: phylum Hepatophyta (the liverworts), phylum Anthocerophyta (the hornworts), and phylum Bryophyta (the mosses).

Origin and Relationships

Bryophytes are thought to have originated more than 430 million years ago, during the Silurian period. Many botanists speculate that bryophytes arose from an ancestor in the green algal order Charales or Coleochaetales based on biochemical, morphological, and life history comparisons.

For example, Chara has a flavonoid biosynthesis pathway that is similar to that of higher plants, while Coleochaete retains its zygote inside parental tissue, similar to higher plants. These characteristics, along with similarities in cell division patterns, photosynthetic pigment contents, and the use of starch as a storagematerial, all suggest ancestry in the Charales or Coleochaetales orders.


Historically, bryophytes were thought to represent a group that formed a separate lineage from that of vascular plants. By the late 1990’s a growing body of evidence suggested that bryophytes and vascular plants were derived from a common green algal ancestor.

Some botanists suggest that the earliest land plants may have been members of the phylum Anthocerophyta. One of the key arguments in this theory is that the structure of some hornwort chloroplasts is virtually identical to the chloroplast structure of the presumed algal ancestors.

Studies conducted in the 1990’s involving the presence or absence of certain portions of noncoding deoxyribonucleic acid (DNA) called introns in the genetic information of several groups of algae, bryophytes, and vascular plants revealed that members of the Hepatophyta, the liverworts, were likely among the first land plants.

Like the algae, they lack the introns that are found in groups that are presumed to be more derived. Thus, based on the assumption that introns are derived characters, ancestors of modern liverworts may have given rise to vascular plants.

Anatomy

The dominant phase of the bryophyte life cycle is the haploid gametophyte phase. The gametophyte is photosynthetic and is usually small because of the lack of efficient vascular tissues.

Bryophytes possess rootlike rhizoids that anchor the plant to the soil and aid in nutrient uptake. A waxy cuticle, which helps prevent water loss, covers the body. Liverworts have pores for gas exchange, while hornworts and mosses have stomata to regulate gas movement. Some liverworts and hornworts have a thalloid body type,which is not differentiated into leaf and stem.

The thallus may be simple, composed of a ribbon like, flattened body of relatively undifferentiated tissues, or complex, in which there is a distinct differentiation of tissues. The flat body may aid in the uptake of water and minerals and in gas exchange. The bodies of some liverworts and the mosses are divided into leaf and stem. These terms are used for convenience even though xylem and phloem are not present.

Somemosses possess tissues that have functions similar to xylem and phloem. Hydroids are water conducting cells that make up a tissue called hadrom. Leptoids are food-conducting cells that make up a tissue called leptom. These tissues appear similar to the conducting tissues in a group of fossil plants called protracheophytes, which are thought to be an intermediate group between the bryophytes and the vascular plants.

The diploid sporophyte of liverworts and mosses consists of a foot, which is attached to a stalklike seta. The seta connects the foot to the spore-producing organ called the sporangium,or capsule. The hornwort sporophyte, however, lacks a seta and possesses a long, cylindrical sporangium. The foot of the bryophyte sporophyte contains specialized transfer cells, which bring materials from the maternal gametophyte to the sporophyte.

The sporophyte is totally dependent on the maternal gametophyte for its survival. A layer of sterile tissue called the calyptra covers the capsules of liverworts and mosses. When the spores are mature, the sporophytemay die, allowing the release of spores as the capsule decays (as in some thalloid liverworts).

Alternatively, the capsule may rupture, allowing spores to be released through pores (as in mosses and leafy liverworts), or the capsule may split along the side to release the spores (as in the hornworts). Liverworts and hornworts often have specialized structures in the capsules called elaters that aid in dispersing spores from the capsules.

Reproduction and Life Cycle

bryophytes reproduction and life cycle
bryophytes reproduction and life cycle
Bryophytes may reproduce either asexually or sexually. Asexual reproduction primarily occurs by fragmentation. Some of the liverworts also reproduce asexually by the production of small masses of vegetative tissue called gemmae in special structures called gemma cups. Water drops disperse the gemmae.

Bryophytes exhibit a typical plant life-cycle pattern called alternation of generations. There are distinct male and female gametophytes in some species, while other species produce both male and female organs in one plant. The reproductive organs are all multicellular. Male organs are called antheridia.

Special cells within an antheridium undergo mitotic cell division to produce flagellated haploid sperm cells. The sperm cells are the only flagellated cells produced by bryophytes. As with many other plant groups, the presence of flagella on the sperm indicates that these cells require liquid water to swim to the egg.

The female organs are called archegonia. The archegoniumis composed of a slender neck, within which is a canal. The base of the archegonium has a swollen region called the venter, which contains the egg. Special cells within an archegonium undergo mitotic cell division to produce a haploid egg.

If one gametophyte produces both antheridia and archegonia, the organs usually develop at different times, to reduce to likelihood of self-fertilization. When the sperm and eggs are mature, sperm are released from the antheridia in the presence of liquid water.

Water drops transfer sperm from an antheridium to an archegonium. Sperm cells swim through the neck canal of the archegonium where fertilization occurs in the venter. The resulting zygote develops into an embryo, which then grows into the diploid sporophyte.

Sporogenous tissues in the sporangium undergo meiosis to produce haploid spores. The spore walls contain a substance called sporopollenin, which is resistant to chemicals and decay. After release, spores germinate and grow into new haploid gametophytes. The early threadlike stage of mosses and some liverworts is called the protonema. Protonemata are very similar to the body form of some algae.

Phylum Hepatophyta

There are between six thousand and eight thousand species of hepatophytes (from the Greek word hepar, meaning “liver”), which are commonly called liverworts. Hepatophytes are divided into three general groups: the simple thalloid liverworts, the complex thalloid liverworts, and the leafy liverworts. More than 85 percent of all hepatophyte species are leafy.

Liverworts are usually terrestrial, although some species may be semi aquatic. Thalloid types are found worldwide. Leafy liverworts, which are often similar in appearance to mosses, are abundant in tropical jungles and fog belts. However, they are typically found in habitats that are more moist than those preferred by mosses.

Phylum Anthocerophyta

This phylum, the hornworts, consists of some one hundred species and represents the smallest group of bryophytes. The best-known genus, Anthoceros (from the Greek words anthos, meaning “flower” and keras, meaning “horn”), is found in temperate regions. The gametophyte is similar to thalloid liverworts. The cavities of the gametophyte body are filled with mucilage, a slimy secretion, in which grow nitrogen-fixing cyanobacteria, such as the genus Nostoc.

Phylum Bryophyta

Phylum Bryophyta, the mosses, consists of more than ninety-five hundred species. There are three important classes: class Sphagnidae, which includes the globally distributed, and economically as well as ecologically important genus Sphagnum; class Andreaeidae, which consists of a small group of blackish green to reddish brown tufted rock mosses growing on granitic or calcareous rocks in northern latitudes; and the class Bryidae, which consists of true mosses.

Economic Uses

Bryophytes are ecologically important members of terrestrial ecosystems. They are primary producers, providing food and habitat for animals. Humans have used bryophytes for many purposes. For example, Sphagnum deposits in peat bogs have been used for centuries as fuel for heating and cooking.

Dried Sphagnum also has the ability to absorb large amounts of liquid, which makes it ideal to act as a soil conditioner for planting. American Indians used mosses as compresses to dress wounds. The antiseptic quality of Sphagnum, along with its absorptive properties, made its use attractive as bandage material for the British when cotton supplies were low during World War I.
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1:51 AM
Bulbs and rhizomes are modified stems, stem bases, or other underground organs used by plants for food (or energy) storage and in asexual reproduction.

Plants reproduce both sexually and asexually. Although sexual reproduction is part of the typical life cycle of plants, for a variety of reasons a plant may reproduce asexually. Exact duplicates of a plant, called clones, are formed by asexual reproduction.

Asexual Reproduction

Asexual reproduction involves the production of offspring through the formation of propagules by mitosis (the process of nuclear cell division). Because genetic recombination does not occur in mitosis, the offspring are genetically identical to the parent plant.

Asexual reproduction does not occur in all plants; some reproduce asexually only when humans intervene. Asexual reproduction occurs when a single plant produces a vegetative propagule that develops into a separate free-living plant. Many of the propagules that support asexual reproduction are actually highly modified branches. Others are modified roots.


In rare instances, the tissues of leaves may be modified by nature to support asexual reproduction. The propagules of asexual reproduction vary enormously. They are often found in catalogs describing “bulbs,” but technically they include true bulbs, corms, stolons, tubers, rhizomes, turions, pseudobulbs, and fleshy roots.

True Bulbs

Bulbs, corms, stolons, tubers, rhizomes, and turions are all modified stems. Bulbs are modified stem bases that develop underground. The stem is shortened and thickened to produce a mass of tissue shaped like a coin or like a child’s toy top. Scale like leaves with thickened bases are attached to the base of the bulb.

Starch is stored in the thickened bases, a food supply that allows the bulb to survive through a dormant season and to produce adventitious roots. Roots are often absent when the bulb is dormant. The starch can also support a period of rapid stem and leaf growth in the growing season and may support the flowering and fruiting of the plant.

In tunicate bulbs, a cloak of dried leaves surrounds the outside of the bulb. These dried leaves provide a barrier to desiccation and allow the tunicate bulbs to be stored above ground for weeks or even months.

Onions (Allium cepa) and daffodils (Narcissus) are examples of tunicate bulbs. Other bulbs have no cloak and usually have shorter, less cylindrical leaves. These scaly bulbs dry quickly when kept above ground and usually develop flowers only after a more normal, aerial branch system forms. Lilies (Lilium) have scaly bulbs.

Stems of both tunicate and scaly bulbs can branch. Below ground, branches appear at first as miniature bulbs (bulbils or bulblets). Bulblets take their energy from the parent bulb but eventually produce aerial stems or leaves and can be separated from the parent.

Profuse branching can be stimulated by wounding the stem of the parent bulb. All the bulbs produced by this technique are clones, identical to the parent in their genetic makeup and physical characteristics.

Corms and Tubers

Corms are similar to bulbs in many ways. They have a disk-shaped or top-shaped stem mass that is shorter and wider than most typical stems. They are often cloaked in a tunic of dried leaves that are thinner and smaller than those on bulbs.

Corms do not store significant amounts of starch; it is instead stored primarily in the basal plate of the stem. Branches of the corm stem produce new, miniature corms (cormels). Wounding the parent stem stimulates greater branching. Gladiolus and crocus are two common garden plants that produce corms.

Tubers are thick, starchy stems that form usually at the tip of a stolon, runner, or tiller. Tubers may form on the soil surface or below ground. A familiar example is the white or Irish potato (Solanum tuberosum). The leaves on most tubers are much smaller than the leaves on other parts of the stem, but above each leaf on the tuber is a well developed axillary bud, commonly called an eye.

The axillary bud has the potential to elongate, forming a complete and fully developed branch. If the stolon connecting the tuber to the parent plant dies, the branch from the eye of the tuber becomes an independent clone of the parent plant.

Most tubers contain many eyes. If the tuber is cut into smaller pieces, each containing an eye, each piece develops a rhizome, which in turn develops a newtuber. By this technique, a significant increase in the number of plants can be obtained.

The cut pieces of tuber are initially prone to decay, but after they have dried for a few days, they heal over with a layer of callus which protects them like a skin. Cutting tubers into small “seed” pieces is a common method for propagating tuber-forming species.

Rhizomes, Stolons, and Fleshy Roots

Rhizomes are specialized, underground stems. Unlike most areal stems, the rhizomes are normally oriented horizontally. Just as pieces of tuber can provide the tissue and energy source for the formation of a new plant, so too can pieces of a rhizome.

Many ferns and fern allies propagate naturally by rhizomes. Large stands of these plants can form from a single individual as the rhizomes grow and branch. Eventually, older pieces of the rhizome die, leaving a population of individuals that all have identical genetic characteristics.

Stolons are long, thin, horizontal stems, also called runners or tillers, which grow along the surface of the ground. When the stolon has grown far enough from the parent plant, the growth pattern changes, and a crown, or tuber, forms. A crown is a compressed stem mass with leaves arranged close to one another, also called a rosette. Within the crown, roots format the points of attachment of the leaves to the stems.

If the stolon is broken or dies, the crown becomes an independent clone of the parent plant. In this way, a large number of off-spring can be produced from a single plant. This is a mechanism of reproduction of the strawberry (Fragaria) and is also a common reproductive mechanism for grasses, including crab grass (Digitaria sanguinalis) and quack grass (Agropyron repens).

Fleshy roots store energy that can be useful for asexual propagation. Most require at least a small amount of stem tissue to support cell growth and differentiation. The true yam (Dioscorea) is a tuber, but the sweet potato (Ipomoea batatas) is a fleshy root which can easily be propagated asexually. Many buttercups (Ranunculus) are also propagated by breaking up the clumps of their fleshy roots.
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11:45 PM

Alternative forms of photosynthesis are used by specific types of plants, called C4 and CAM plants, to alleviate problems of photorespiration and excess water loss.

Photosynthesis is the physiological process whereby plants use the sun’s radiant energy to produce organic molecules. The backbone of all such organic compounds is a skeleton composed of carbon atoms. Plants use carbon dioxide from the atmosphere as their carbon source.

The overwhelming majority of plants use a single chemical reaction to attach carbon dioxide from the atmosphere onto an organic compound, a process referred to as carbon fixation. This process takes place inside specialized structures within the cells of green plants known as chloroplasts.


The enzyme that catalyzes this fixation is ribulose bisphosphate carboxylase (Rubisco), and the first stable organic product is a three-carbon molecule. This three-carbon compound is involved in the biochemical pathway known as the Calvin cycle. Plants using carbon fixation are referred to as C3 plants because the first product made with carbon dioxide is a three-carbon molecule.

C4 Photosynthesis

For many years scientists thought that the only way photosynthesis occurred was through C3 photosynthesis. In the early 1960’s, however, researchers studying the sugarcane plant discovered a biochemical pathway that involved incorporation of carbon dioxide into organic products at two different stages.

First, carbon dioxide from the atmosphere enters the sugarcane leaf, and fixation is accomplished by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase). This step takes place within the cytoplasm, not inside the chloroplasts. The first stable product is a four-carbon organic compound that is an acid, usually malate. Sugarcane and other plants with this photosynthetic pathway are known as C plants.

In C4 plants, this photosynthetic pathway is tied to a unique leaf anatomy known as Kranz anatomy. This term refers to the fact that in C4 plants the cells that surround the water- and carbohydrate conducting system (known as the vascular system) are packed very tightly together and are called bundle sheath cells.

Surrounding the bundle sheath is a densely packed layer of mesophyll cells. The densely packed mesophyll cells are in contact with air spaces in the leaf, and because of their dense packing they keep the bundle sheath cells from contact with air. This Kranz anatomy plays a major role in C4 photosynthesis.

In C4 plants the initial fixation of carbon dioxide from the atmosphere takes place in the densely packed mesophyll cells. After the carbon dioxide is fixed into a four-carbon organic acid, the malate is transferred through tiny tubes from these cells to the specialized bundle sheath cells.

Inside the bundle sheath cells, the malate is chemically broken down into a smaller organic molecule, and carbon dioxide is released. This carbon dioxide then enters the chloroplast of the bundle sheath cell and is fixed a second timewith the enzyme Rubisco and continues through the C3 pathway.

Advantages of Double-Carbon Fixation

The double-carbon fixation pathway confers a greater photosynthetic efficiency on C4 plants over C3 plants, because the C3 enzyme Rubisco is highly inefficient in the presence of elevated levels of oxygen. In order for the enzyme to operate, carbon dioxide must first attach to the enzyme at a particular location known as the active site.

However, oxygen is also able to attach to this active site and prevent carbon dioxide from attaching, a process known as photorespiration. As a consequence, there is an ongoing competition between these two gases for attachment at the active site of the Rubisco enzyme. Not only does the oxygen outcompete carbon dioxide; when oxygen binds to Rubisco, it also destroys some of the molecules in the Calvin
cycle.

At any given time, the winner of this competition is largely dictated by the relative concentrations of these two gases. When a plant opens its stomata (the pores in its leaves), the air that diffuses in will be at equilibrium with the atmosphere, which is 21 percent oxygen and 0.04 percent carbon dioxide.

During hot, dry weather, excess water vapor diffuses out, and under these conditions plants face certain desiccation if the stomata are left open continuously.When these pores are closed, the concentration of gases will change. As photosynthesis proceeds, carbon dioxide will be consumed and oxygen generated.

When the concentration of carbon dioxide drops below 0.01 percent, oxygen will outcompete carbon dioxide at the active site, and no net photosynthesis occurs. C4 plants, however, are able to prevent photorespiration, because the PEP carboxylase enzyme is not inhibited by oxygen.

Thus, when the stomata are closed, this enzyme continues to fix carbon inside the leaf until it is consumed. Because the bundle sheath is isolated from the leaf’s air spaces, it is not affected by the rising oxygen levels, and the C3 cycle functions without interference. C4 photosynthesis is found in at least nineteen families of flowering plants.

No family is exclusively composed of C4 plants. Because C4 photosynthesis is an adaptation to hot, dry environments, especially climates found in tropical regions, C4 plants are often able to out compete C3 plants in those areas. In more temperate regions, they have less of an advantage and are therefore less common.

CAM Photosynthesis


A second alternative photosynthetic pathway, known as crassulacean acid metabolism (CAM), exists in succulents such as cacti and other desert plants. These plants have the same two carbon-fixing steps as are present in C4 plants, but rather than being spatially separated between the mesophyll and bundle sheath cells, CAM plants have both carbon dioxide-fixing enzymes within the same cell.

These enzymes are active at different times, PEP carboxylase during the day and Rubisco at night. Just as Kranz anatomy is unique to C4 plants, CAM plants are unique in that the stomata are open at night and largely closed during the day.

The biochemical pathway of photosynthesis in CAM plants begins at night. With the stomata open, carbon dioxide diffuses into the leaf and into mesophyll cells, where it is fixed by the C4 enzyme PEP carboxylase. The product is malate, as in C4 photosynthesis, but it is transformed into malic acid (a nonionic form of malate) and is stored in the cell’s vacuoles (cavities within the cytoplasm) until the next day.

Although the malic acid will be used as a carbon dioxide source for the C3 cycle, just as in C4 photosynthesis, it is stored until daylight because the C3 cycle requires light as an energy source. The vacuoles will accumulate malic acid through most of the night.

A few hours before daylight, the vacuole will fill up, and malic acid will begin to accumulate in the cytoplasm outside the vacuole. As it does, the pH of the cytoplasm will become acidic, causing the enzyme to stop functioning for the rest of the night.

When the sun rises the stomata will close, and photosynthesis by the C3 cycle will quickly deplete the atmosphere within the leaf of all carbon dioxide. At this time, the malic acid will be transported out of the vacuole to the cytoplasmof the cell. There it will be broken down, and the carbon dioxide will enter the chloroplast and be used by the C3 cycle; thus, photosynthesis is able to continue with closed stomata.

Crassulacean acid metabolism derives its name from the fact that it involves a daily fluctuation in the level of acid within the plant and that it was first discovered to be common in species within the stonecrop family, Crassulaceae.

The discovery of this photosynthetic pathway dates back to the 1960’s. The observation that succulent plants become very acidic at night, however, dates back to at least the seventeenth century, when it was noted that cactus tastes sour in the morning and bitter in the afternoon.

CAM Plant Ecosystems

There are two distinctly different ecological environments where CAM plants may be found. Most are terrestrial plants typical of deserts or other harsh, dry sites.

In these environments, the pattern of stomatal opening and closing provides an important advantage for surviving arid conditions: When the stomata are open, water is lost; however, the rate of loss decreases as the air temperature decreases. By restricting the time period of stomatal opening to the nighttime, CAM plants are extremely good at conserving water.

The other ecological setting where CAM plants are found is in certain aquatic habitats. When this environment was first discovered, it seemed quite odd, because in these environments conserving water would be of little value to a plant. It was found, however, that there are aspects of the aquatic environment which make CAM photosynthesis advantageous.

In shallow bodies of water, the photosynthetic consumption of carbon dioxide may proceed at a rate in excess of the rate of diffusion of carbon dioxide from the atmosphere into the water, largely because gases diffuse several times more slowly in water than in air.

Consequently, pools of water may be completely without carbon dioxide for large parts of the day. Overnight, carbon dioxide is replenished, and aquatic CAM plants take advantage of this condition to fix the plentiful supply of carbon dioxide available at night and store it as malic acid.

Hence, during the day, when the ambient carbon dioxide concentration is zero, these plants have their own internal supply of carbon dioxide for photosynthesis. Thus, two very different ecological conditions have selected for the identical biochemical pathway.

These two modified photosynthetic pathways adequately describe what happens in most terrestrial plants, although there is much variation. For example, there are species that appear in many respects to have photosynthetic characteristics intermediate to C3 and C4 plants.

Other plants are capable of switching from exclusively C3 photosynthesis to CAM photosynthesis at different times of the year. Photosynthesis by aquatic plants appears to present even more variation. C3-C4 intermediate plants seem to be relatively common compared to the terrestrial flora, and several species have C4 photosynthesis but lack Kranz anatomy.
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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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