NATURAL SELECTION AND EVOLUTION

NATURAL SELECTION AND EVOLUTION

A.    The Role Of Natural Selection

Evolution is the continuous genetic adaptation of a population of organisms to its environment over time. Evolution results when there are changes in genes present in a population. There are three factors that interact to determine how a species changes over time: environmental factors that affect organisms, sexual reproduction among the individuals in the gene pool, and the generation of genetic variety within the gene pool.

The idea that some individuals whose gene combinations favor life in their surroundings will be most likely to survive, reproduce, and pass their genes on to the next generation is known as the theory of natural selection. The theory of natural selection was first proposed by Charles Darwin and Alfred Wallace and was clearly set forth in 1859 by Darwin in his book On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life

B.     What Influences Natural Selection ?

Spontaneous mutations are changes in DNA that cannot be tied to a particular causative agent. It is suspected that cosmic radiation or naturally occurring mutagenic chemicals might be the cause of many of these mutations. It is knownthat subjecting organisms to high levels of radiation or to certain chemicals increases the rate at which mutationsoccur. In order for mutations to be important in the evolution of organisms, they must be in cells that will become gametes. Mutations to the cells of the skin or liver will only affect those specific cells and will not be passed on to the next generation.

A second very important process involved in generating genetic variety is sexual reproduction. Although sexual reproduction does not generate new genetic information the way mutations do, it allows for the recombination of genes into mixtures that did not occur previously. Each individual entering a population by sexual reproduction carries a unique combination of genes; approximately half donated by the mother and half donated by the father. During meiosis, variety is generated in the gametes through crossing-over between homologous chromosomes and independent assortment of nonhomologous chromosomes. This results in millions of possible combinations of genes in the gametes of any individual. When fertilization occurs, one of the millions of possible sperm unites with one of the millions of possible eggs, resulting in a genetically unique individual. The gene mixing that occurs during sexual reproduction is known as genetic recombination. The new individual has a complete set of genes that is different from that of any other organism that ever existed.

The importance of generating new gene combinations is particularly important because the way genes express themselves in an individual can depend on the other genes present. Genes don’t always express themselves in the same way. In order for genes to be selected for or against, they must be expressed in the phenotype of the individuals possessing them. There are many cases of genes expressing themselves to different degrees in different individuals. Often the reason for this difference is unknown. Penetrance is a term used to describe how often an allele expresses itself when present. Expressivity is a term used to describe situations in which the gene expresses itself but not equally in all individuals that have it.

Whenever a successful organism is examined, it can be shown that it reproduces at a rate in excess of that necessary to merely replace the parents when they die. Although the total number of organisms in the species may remain constant, the individuals that make up the population change. It is this extravagant reproduction that provides the large surplus of genetically different individuals that allows natural selection to take place. In fact, to maintain itself in an ever-changing environment, each species must change in ways that enhance its ability to adapt to its new environment. For this to occur, members of the population must be eliminated in a non-random manner. Those individuals that survive are those that are, for the most part, better suited to the environment than other individuals. They reproduce more of their kind and transmit more of their genes to the next generation than do individuals with genes that do not allow them to be well adapted to the environment in which they live.

C.    Common Misunderstandings About Natural Selection

There are several common misinterpretations associated with the process of natural selection. The first involves the phrase “survival of the fittest.” Individual survival is certainly important because those that do not survive will not reproduce. But the more important factor is the number of descendants an organism leaves.

Second, the phrase “struggle for life” does not necessarily refer to open conflict and fighting. It is usually much more subtle than that. When a resource such as nesting material, water, sunlight, or food is in short supply, some individuals survive and reproduce more effectively than others.

A third common misunderstanding involves significance of phenotypic characteristics that are not caused by genes. Many organisms survive because they have characteristics that are not genetically determined. The acquired characteristics are gained during the life of the organism; they are not genetically determined and, therefore, cannot be passed on to future generations through sexual reproduction. Therefore, acquired characteristics are not important to the processes of natural selection.

D.    Proccess That Drive Natural Selection

Differential Survival

As stated previously, the phrase “survival of the fittest” is often associated with the theory of natural selection. Although this is recognized as an oversimplification of the concept, survival is an important factor in influencing the flow of genes to subsequent generations. If a population consists of a large number of genetically and phenotypically different individuals it is likely that some of them will possess characteristics that make their survival difficult. Therefore, they are likely to die early in life and not have an opportunity to pass their genes on to the next generation.

Differential Reproductive Rates

Survival alone does not always ensure reproductive success. For a variety of reasons, some organisms may be better able to utilize available resources to produce offspring. If one individual leaves 100 offspring and another leaves only 2, the first organism has passed more copies of its genetic information information on to the next generation than has the second. If we assume that all 102 individual offspring have similar survival rates, the first organism has been selected for and its genes have become more common in the subsequent population.

            Differential Mate Selection

Within animal populations, some individuals may be chosen as mates more n others. This is called “sexual selection.” Obviously, those that are frequently chosen have an opportunity to pass on more copies of their genes than those that are rarely chosen. Characteristics of the more frequently chosen individuals may involve general characteristics, such as body size or aggressiveness, or specific conspicuous characteristics attractive to the opposite sex.

E.     Gene-Frequency Studies and Hardy-Weinberg Equilibrium

In the early 1900s an English mathematician, G. H. Hardy, and a German physician, Wilhelm Weinberg, recognized that it was possible to apply a simple mathematical relationship to the study of gene frequencies. Their basic idea was that if certain conditions existed, gene frequencies would remain constant, and that the distribution of genotypes could be described by the relationship A2 + 2Aa + a2 = 1, where A2 represents the frequency of the homozygous dominant genotype, 2Aa represents the frequency of the heterozygous genotype, and a2 represents the frequency of the homozygous recessive genotype. Constant gene frequencies over several generations would imply that evolution is not taking place. Changing gene frequencies would indicate that evolution is taking place.

The conditions necessary for gene frequencies to remain constant are:

1. Mating must be completely random.

2. Mutations must not occur.

3. Migration of individual organisms into and out of the population must not occur.

4. The population must be very large.

5. All genes must have an equal chance of being passed on to the next generation. (Natural selection is not occurring.)

The concept that gene frequencies will remain constant if these five conditions are met has become known as the Hardy-Weinberg concept. The Hardy-Weinberg concept is important because it allows a simple comparison of allele frequency to indicate if genetic changes are occurring. Two different populations of the same organism can be compared to see if they have the same allele frequencies, or populations can be examined at different times to see if allele frequencies are changing.

F.     A Summary Of The Causes Of Evolutionary Changes

Evolution was described as the change in gene frequency over time. We can now see that several different mechanisms operate to bring about this change. Mutations can either change one allele into another or introduce an entirely new piece of genetic information into the population. Immigration can introduce new genetic information if the entering organisms have unique genes. Emigration and death remove genes from the gene pool. Natural selection systematically filters some genes from the population, allowing other genes to remain and become more common. The primary mechanisms involved in natural selection are differences in deathrates, reproductive rates, and the rate at which individuals are selected as mates. In addition, gene frequencies are more easily changed in small populations because events such as death, immigration, emigration, and mutation can have a greater impact on a small population than on a large population.

2.      SPECIATION AND EVOLUTIONARY CHANGE

A.    Species : A Working Definintion

A species is commonly defined as a population of organisms whose members have the potential to interbreed naturally to produce fertile offspring but do not interbreed with other groups. This is a working definition; it applies in most cases but must be interpreted to encompass some exceptions. There are two key ideas within this definition. First, a species is a population of organisms. Second, the definition involves the ability of individuals within the group to produce fertile offspring. Obviously, we cannot check every individual to see if it is capable of mating with any other individual that is similar to it, so we must make some judgment calls.

B.     How New Species Originate

Geographic Isolation

The geographic area over which a species can be found is known as its range. The range of the human species is the entire world. As a species expands its range or environmental conditions change in some parts of the range, portions of the population can become separated from the rest. Thus, many species consist of partially isolated populations that display characteristics significantly different from other local populations. Many of the differences observed may be directly related to adaptations to local environmental conditions. This means that new colonies or isolated populations may have infrequent gene exchange with their geographically distant relatives.

Speciation is the process of generating new species. This process has occurred only if gene flow between isolated populations does not occur even after barriers are removed. the process of speciation can begin with the geographic isolation of a portion of the species, but new species are generated only if isolated populations become separate from one another genetically. Speciation by this method is really a three-step process. It begins with geographic isolation, is followed by the action of selective agents that choose specific genetic combinations as being valuable, and ends with the genetic differences becoming so great that reproduction between the two groups is impossible.

It is also possible to envision ways in which speciation could occur without geographic isolation being necessary. Any process that could result in the reproductive isolation of a portion of a species could lead to the possibility of speciation.

            Polyploidy: Instant Speciation

Polyploidy is a condition of having multiple sets of chromosomes rather than the normal haploid or diploid number. The increase in the number of chromosomes can result from abnormal mitosis or meiosis in which the chromosomes do not separate properly. Because the number of chromosomes of the polyploid is different from that of the parent, successful reproduction with the parent species would be difficult. This is because meiosis would result in gametes that had different chromosome numbers from the original, parent organism. In one step, the polyploid could be isolated reproductively from its original species.

C.    Maintaining Genetic Isolation

In order for a new species to continue to exist, it must reproduce but continue to remain genetically distinct from other similar species. The speciation process typically involves the development of reproductive isolating mechanisms or genetic isolating mechanisms. These mechanisms prevent matings between species and therefore help maintain distinct species. A great many types of genetic isolating mechanisms are recognized.

D.    The Development Of Evolutionary Thought

In the mid-1700s, Georges-Louis Buffon, a French naturalist, expressed some curiosity about the possibilities of change (evolution) in animals, but he did not suggest any mechanism that would result in evolution. In 1809, Jean-Baptiste de Lamarck, a student of Buffon’s, suggested a process by which evolution could occur. He proposed that acquired characteristics could be transmitted to offspring. Although we now know Lamarck’s theory was wrong (because acquired characteristics are not inherited), it stimulated further thought as to how evolution could occur.

In 1858, Charles Darwin and Alfred Wallace suggested the theory of natural selection as a mechanism for evolution. They based their theory on the following assumptions about

the nature of living things:

1. All organisms produce more offspring than can survive.

2. No two organisms are exactly alike.

3. Among organisms, there is a constant struggle for survival.

4. Individuals that possess favorable characteristics for their environment have a higher rate of survival and produce more offspring.

5. Favorable characteristics become more common in the species, and unfavorable characteristics are lost.

This logic seems simple and obvious today, but remember that at the time Darwin and Wallace proposed their theory, the processes of meiosis and fertilization were poorly understood, and the concept of the gene was only beginning to be discussed. Nearly 50 years after Darwin and Wallace

suggested their theory, the rediscovery of the work of Gregor Mendel (chapter 10) provided an explanation for how characteristics could be transmitted from one generation to the next.Not only did Mendel’s idea of the gene provide ameans of passing traits from one generation to the next, it also provided the first step in understanding mutations, gene flow, and the significance of reproductive isolation. All of these ideas are interwoven into the modern concept of evolution.

E.     Evolutionary Patterns Above The Species Level

The basic evolutionary pattern is one of divergent evolution in which individual speciation events cause successive branches in the evolution of a group of organisms. Furthermore, it is important to recognize that many extinct species were very successful organisms for millions of years. They were not failures for their time but simply did not survive to the present. It is also important to realize that many currently existing organisms will  entually become extinct.

Although divergence is the basic pattern in evolution, it is possible to superimpose several other patterns on it. One special evolutionary pattern, characterized by a rapid increase in the number of kinds of closely related species, is known as adaptive radiation. Adaptive radiation results in an evolutionary explosion of new species from a common ancestor. There are basically two situations that are thought to favor adaptive radiation. One is a condition in which an organism invades a previously unexploited environment. A second set of conditions that can favor adaptive radiation is one in which a type of organism evolves a new set of characteristics that enable it to displace organisms that previously filled roles in the environment.

Another evolutionary pattern, convergent evolution, occurs when organisms of widely different backgrounds develop similar characteristics. This particular pattern often leads people to misinterpret the evolutionary history of organisms.

F.     Rates Of Evolution

When we examine the fossil record, we can often see gradual changes in physical features of organisms over time. This is such a common feature of the evolutionary record that biologists refer to this kind of evolutionary change as gradualism. Charles Darwin’s view of evolution was based on gradual changes in the features of specific species he observed in his studies of geology and natural history.

However, as early as the 1940s, some biologists began to challenge gradualism as the typical model for evolutionary change. They pointed out that the fossils of some species were virtually unchanged over millions of years. If gradualism were the only explanation for how species evolved, then

gradual changes in the fossil record of a species would always be found. Furthermore, some organisms appear suddenly in the fossil record and show rapid change from the time they first appeared.

In 1972, two biologists, Niles Eldredge of the American Museum of Natural History and Stephen Jay Gould of Harvard University, proposed the idea of punctuated equilibrium. This hypothesis suggests that evolution occurs in spurts of rapid change followed by long periods with little evolutionary change. It is important to recognize that the punctuated equilibrium concept suggests a different way of achieving evolutionary change.

G.    The Tentative Nature of The Evolutionary History Of Organisms

It is important to understand that thinking about the concept of evolution can take us in several different directions. First, it is clear that genetic changes do occur. Mutations introduce new genes into a species. This has been demonstrated repeatedly with chemicals and radiation. We also recognize that species can change. We purposely manipulate the genetic constitution of our domesticated plants and animals and change their characteristics to sui our needs. We also recognize that different populations of the same species show genetic differences. Examination of fossils shows that species of organisms that once existed are no longer in existence. We can also demonstrate that new species come into existence. This is easiest to do in plants with polyploidy. It is clear from this evidence that species are not fixed, unchanging entities.

However, the fossil record is not a complete record and new fossils are being discovered every year. There are several reasons why the fossil record is incomplete. First of all the likelihood that an organism will become a fossil is low. Most organisms die and decompose leaving no trace of their existence. In addition, some organisms have very resistant parts that tend to be preserved while others do not. Clams and insects are abundant in the fossil record. Worms are not. Finally, the discovery of fossils is often accidental. It is impossible to search through all the layers of sedimentary rock on the entire surface of the Earth.

H.    Human Evolution

Various species of Australopithecus and Paranthropus were present in Africa from about 4.4 million years ago untilabout 1 million years ago. However, from examining the fossil bones of the leg, pelvis, and foot, it is apparent that the australopiths were relatively short (males, 1.5 meters or less; females, about 1.1 meters) and stocky and walked upright like humans.

About 2.5 million years ago the first members of the genus Homo appeared on the scene. There is considerable disagreement about how many species there were but Homo habilis is one of the earliest. Homo habilis had a larger brain (650 cubic centimeters) and smaller teeth than australopiths and made much more use of stone tools.

About 1.8 million years ago Homo ergaster appeared on the scene. It was much larger (up to 1.6 meters) than H. habilis (about 1.3 meters) and also had a much larger brain (cranial capacity of 850 cubic centimeters). A little later a similar species (Homo erectus) appears in the fossil record.

About 800,000 years ago another hominid, classified as Homo heidelbergensis, appears in the fossil record. Since fossils of this species are found in Africa, Europe, and Asia, it appears that they constitute a second wave of migration of early Homo from Africa to other parts of the world.

There are two different theories that seek to explain the origin of Homo sapiens.One theory, known as the out-of-Africa hypothesis, states that modern humans (Homo sapiens) originated in Africa as had several other hominid species and migrated from Africa to Asia and Europe and displaced species such as H. erectus and H. Heidelbergensis that had migrated into these areas previously. The other theory, known as the multiregional hypothesis, states that H. erectus evolved into H. sapiens. During a period of about 1.7 million years, fossils of Homo erectus showed a progressive increase in the size of the cranial capacity and reduction in the size of the jaw, so that it becomes difficult to distinguish H. erectus from H. heidelbergensis and H. Heidelbergensis from H. sapiens. Proponents of this hypothesis believe that H. heidelbergensis is not a distinct species but an intermediate between the earlier H. erectus and H. sapiens.

The early evolution of humans has been difficult to piece together because of the fragmentary evidence. Beginning about 4.4 million years ago the earliest forms of Australopithecus and Paranthropus showed upright posture and other humanlike characteristics.

The structure of the jaw and teeth indicates that the various kinds of australopiths were herbivores. Homo habilis had a larger brain and appears to have been a scavenger. Several other species of the genus Homo arose in Africa. These forms appear to have been carnivores. Some of these migrated to Europe and Asia.The origin of Homo sapiens is in dispute. It may have arisen inAfrica and migrated throughout the world or evolved from earlier ancestors found throughout Africa, Asia, and Europe.

3.      THE PLANT BODY

The plant body creates and maintains an internal environment that differs from the external environment. Plants accomplish through growth some of the same things that animals achieve through mobility. they must nevertheless obtain nutrients not only the raw materials of photosynthesis (carbon dioxide and water), but also mineral elements such as nitrogen, potassium, and calcium. Seed plants—even the tallest trees—transport water and minerals from the soil to their tops, and they transport the products of photosynthesis from the leaves to their roots and other parts.

a.      Vegetative Organs of the Flowering Plant Body

Flowering plants (angiosperms) are tracheophytes that are characterized by double fertilization, a triploid endosperm, and seeds enclosed in modified leaves called carpels. Their xylem contains cells

called vessel elements and fibers, and their phloem contains sieve tube elements and companion cells.

Flowering plants possess three kinds of vegetative (nonreproductive) organs: roots, stems, and leaves. Flowers, which are the plant’s devices for sexual reproduction, consist of modified leaves and stems; flowers will be considered in detail in a later chapter. Most flowering plants belong to one of two major lineages. Monocots are generally narrow-leaved flowering plants such as grasses, lilies, orchids, and palms. Eudicots are broad-leaved flowering plants such as soybeans, roses, sunflowers, and maples.

In both lineages, the vegetative plant body consists of two systems: the shoot system and the root system. The shoot system of a plant consists of the stems, leaves, and flowers. Broadly speaking, the leaves are the chief organs of photosynthesis. The stems hold and display the leaves to the sun and provide connections for the transport of materials between roots and leaves. The locations where leaves attach to a stem are called nodes, and the stem regions between successive nodes are internodes. The root system anchors the plant in place and provides nutrition. The extreme branching of plant roots and their high surface area-to-volume ratio allow them to absorb water and mineral nutrients from the soil. Each of the vegetative organs can be understood in terms of its structure. By structure we mean both its overall form, called its morphology, and its component cells and tissues and their arrangement, called its anatomy.

There are two principal types of root systems. Many eudicots have a taproot system: a single, large, deep-growing primary root accompanied by less prominent lateral roots. The taproot itself often functions as a nutrient storage organ, as in carrots. Some plants have adventitious roots. These roots arise above ground from points along the stem; some even arise from the leaves.

Unlike roots, stems bear buds of various types. A bud is an embryonic shoot. A stem bears leaves at its nodes, and where each leaf meets the stem there is a lateral bud. At the tip of each stem or branch is an apical bud, which produces the cells for the upward and outward growth and development of that shoot.

In gymnosperms and most flowering plants, the leaves are responsible for most of the plant’s photosynthesis, producing energy-rich organic molecules and releasing oxygen gas. As photosynthetic organs, leaves are marvelously adapted for gathering light. Typically, the blade of a leaf is a thin, flat structure attached to the stem by a stalk called a petiole. During the daytime, the leaf blade is held by its petiole at an angle almost perpendicular to the rays of the sun.

b.      Plant Cells

Plant cells have all the essential organelles common to eukaryotes they have certain structures and organelles that distinguish them from many other eukaryotes:

_ They contain chloroplasts or other plastids.

_ They contain vacuoles.

_ They possess cellulose-containing cell walls

Cell Wall

       The cytokinesis of a plant cell is completed when the two daughter cells are separated by a cell plate  The daughter cells then deposit a gluelike substance within the cell plate; this substance constitutes the middle lamella. Next, each daughter cell secretes cellulose and other polysaccharides to form a primary wall. This deposition and secretion continue as the cell expands to its final size.Once cell expansion stops, a plant cell may deposit one or more additional cellulosic layers to form a secondary wall internal to the primary wall. Secondary walls are often impregnated with unique substances that give them special properties.

Parenchyma Cell

The most numerous cell type in young plants is the parenchyma cell . Parenchyma cells usually have thin walls, consisting only of a primary wall and the shared middle lamella.The photosynthetic cells in leaves are parenchyma cells that contain numerous chloroplasts. In the cytoplasm of these cells, starch is often stored in specialized plastids called leucoplasts.

Collenchyma Cell

Collenchyma cells are supporting cells. Their primary walls are characteristically thick at the corners of the cells. Collenchyma cells are generally elongated. In these cells, the primary wall thickens, but no secondary wall forms. Collenchyma provides support to leaf petioles, nonwoody stems, and growing organs.

Sclerenchyma Cell

     Sclerenchyma cells have a thickened secondary wall that performs their major function: support. Many sclerenchyma cells function when dead. There are two types of sclerenchyma cells: elongated fibers and variously shaped sclereids. Fibers provide relatively rigid support in wood and other parts of the plant, where they are often organized into bundles. Sclereids may pack together densely.

Xylem

The xylem of tracheophytes conducts water from roots to aboveground plant parts. It contains conducting cells called tracheary elements, which undergo programmed cell death before they assume their function of transporting water and dissolved minerals.

Phloem

     The transport cells of the phloem, unlike those of the mature xylem, are living cells. In flowering plants, the characteristic cells of the phloem are sieve tube elements. These cells meet end-to-end. They form long sieve tubes, which transport carbohydrates and many other materials from their sources to tissues that consume or store them. As sieve tube elements mature, plasmodesmata in their end walls enlarge to form pores, enhancing the connection between neighboring cells. The result is end walls that look like sieves, called sieve plates. At functional maturity, a sieve tube element is filled with sieve tube sap, consisting of water, dissolved sugars, and other solutes

c.       Plant Tissues and Tissue Systems.

Vascular plants have three tissue systems: vascular, dermal, and ground. The vascular tissue system, which includes the xylem and phloem, is the plant’s plumbing or transport system. All the living cells of the plant body require a source of energy and chemical building blocks. The dermal tissue system is the outer covering of the plant. All parts of the young plant body are covered by an epidermis, which may be a single layer of cells or several layers. The epidermis contains epidermal cells and may also include specialized cell types, such as the guard cells that form stomata (pores) in leaves.

The ground tissue system makes up the rest of the plant. It consists primarily of parenchyma tissue, often supplemented by collenchyma or sclerenchyma. Ground tissue functions primarily in storage, support, photosynthesis, and the production of defensive and attractive substances.

d.      Forming the Plant Body

Meristem

There are two types of meristems: Apical meristems give rise to the primary plant body, which is the entire body of many plants.Lateral meristems give rise to the secondary plant body. The stems and roots of some plants (most obviously trees) form wood and become thick; it is the lateral meristems that give rise to the tissues responsible for this thickening.

Both root and shoot apical meristems give rise to a set of cylindrical primary meristems that produce the primary tissues of the plant body.

These complex tissues are derived from two lateral meristems: the vascular cambium and the cork cambium. The vascular cambium is a cylindrical tissue consisting. Without the activity of the cork cambium, this sloughing off of tissues, including the epidermis, would expose the tree to potential damage, including excessive water loss or invasion by microorganisms. The cork cambium produces new protective cells, primarily in the outwarddirection. The walls of these cork cells become impregnated with suberin. predominantly of vertically elongated cells that divide frequently.

The root apical meristem

The root apical meristem produces all the cells that contribute to growth in the length of the root. Some of the daughter cells from the apical (tip) end of the root apical meristem contribute to a root cap, which protects the delicate growing region of the root as it pushes through the soil. Part of the root apical meristem nearest the tip of the root forms a quiescent center, in which cell divisions are rare.

The growing region above the apical meristem comprises the three cylindrical primary meristems: the protoderm, the ground meristem, and the procambium.

The products of the root’s primary meristems become root tissues

The protoderm gives rise to the outer layer of cells the epidermis  which is adapted for protection of the root and for the absorption of mineral ions and water. In the zone of maturation, many of the epidermal cells produce amazingly long, delicate root hairs, which vastly increase the surface area of the root.

Internal to the epidermis, the ground meristem gives rise to a region of ground tissue that is many cells thick, called the cortex. The cells of the cortex are relatively unspecialized and often function in nutrient storage.

Proceeding inward, we come to the endodermis of the root, a single cylindrical layer of cells that is the innermost cell layer of the cortex. Unlike those of other cortical cells, the cell walls of the endodermal cells contain suberin. The placement of this waterproofing substance in only certain parts of the cell wall enables the cylindrical ring of endodermal cells to control the access of water and dissolved ions to the vascular tissues.

Moving inward past the endodermis, we enter the vascular cylinder, or stele, produced by the procambium. The stele consists of three tissues: pericycle, xylem, and phloem

At the very center of the root of a eudicot lies the xylem—seen in cross section in the shape of a star with a variable number of points. Between the points are bundles of phloem. In monocots, a region of parenchyma cells, called the pith, lies in the center of the root. The pith often stores carbohydrate reserves.

The products of the stem’s primary meristems become stem tissues

The primary meristems, in turn, give rise to the three tissue systems. The shoot apical meristem also repetitively lays down the beginnings of leaves and lateral buds.The growing stem has no protective structure analogous to the root cap, but the leaf primordia can act as a protective covering for the shoot apical meristem.

The vascular tissue of a young stem, however, is divided into discrete vascular bundles. Each vascular bundle contains both xylem and phloem. the stem contains other important storage and supportive tissues. Internal to the ring of vascular bundles in eudicots is a storage tissue, the pith, and to the outside lies a similar storage tissue, the cortex.

Many stems and roots undergo secondary growth

Secondary growth results from the activity of the two lateral meristems: vascular cambium and cork cambium. Initially, the vascular cambium is a single layer of cells lying between the primary xylem and the primary phloem. The root or stem increases in diameter when the cells of the vascular cambium divide, producing secondary xylem cells toward the inside of the root or stem and producing secondary phloem cells toward the outside.

As the vascular cambium produces secondary xylem and phloem, its principal cell products are vessel elements, supportive fibers, and parenchyma cells in the xylem and sieve tube elements, companion cells, fibers, and parenchyma cells in the phloem.

Cork, cork cambium, and phelloderm make up the periderm of the secondary plant body. As the vascular cambium continues to produce secondary vascular tissue, the corky layers are in turn lost, but the continuous formation of new cork cambia in the underlying phloem gives rise to new corky layers.

e.       Leaf Anatomy Supports Photosynthesis

Most eudicot leaves have two zones of photosynthetic parenchyma tissue referred to as mesophyll, which means “middle of the leaf.” Within the mesophyll is a great deal of air space through which carbon dioxide can diffuse to reach and be absorbed by photosynthesizing cells.

Vascular tissue branches extensively throughout the leaf, forming a network of veins. Veins extend to within a few cell diameters of all the cells of the leaf, ensuring that the mesophyll cells are well supplied with water and minerals. The products of photosynthesis are loaded into the phloem of the veins for export to the rest of the plant. Guard cells are modified epidermal cells that change their shape, thereby opening or closing pores called stomata, which serve as passageways between the environment and the leaf’s interior. When the stomata are open, carbon dioxide can enter and oxygen can leave, but water vapor can also be lost.