Plant Anatomy and The Primary Plant Body
|Biology - Plant Anatomy|
A seed planted in soil will soon germinate, forming a tiny new root and shoot. The cells of this new plant have been produced by the apical meristems near the ends of the root and shoot. There the cells divide, elongate, and differentiate, forming tissues and tissue systems. That part of the plant formed as a result of the activity of the apical meristems is termed the primary plant body.
Later in the development of many plants, cells are formed from secondary or lateral meristems located parallel to the long axis of the plant. The cells produced by these meristems add bulk to the plant and cause it to increase in diameter. Thus, a secondary plant body composed of secondary tissues is added to the primary. We will examine the primary tissues and tissue systems present in the root, stem, and leaves of the primary vegetative plant body. Let's start with the root.
Put a shovel in the earth almost anywhere on this planet and you will turn up the roots of some plant. Roots serve to take up water and minerals from the soil and conduct them to the stem. Roots also anchor the plant and may also function in storing foodstuffs that are important in the survival of the plant under certain conditions. Most dicotyledons and gymnosperms have a root system characterised by a large, dominant vertical root, known as a taproot.
Grasses and many other monocotyledons, as well as many dicotyledons, have a second type of root system, called a fibrous root system. Here, the primary root is soon replaced by other roots that grow from the stem at or near the soil surface. These are called adventitious roots.
The root system of any given plant is largely an adaptation to the plant's habitat. Apple trees and corn have root systems that both go deeply into the soil and also spread out shallowly just below the surface. This system appears well suited for habitats having moderate rainfall. Many cacti, on the other hand, have extremely shallow but widespread roots, which soak up as much water as possible from the infrequent rain.
An extensive and shallow root system is also found in the coast redwood, which is not a desert dweller, but lives in the damp coastal forests of northern California. Much of the precipitation in these forests is in the form of fog, which condenses against the crowns of the trees and drips to earth, where it is absorbed by the redwood roots lying just under the soil surface.
The growth in length of a root occurs through the division and elongation of cells located in the apical portion of the root. The extreme tip of the root is covered by a mass of cells called the root cap. The cells of the root cap are parenchyma, and frequently have thick, mucilage-containing walls. The root cap protects the root meristem and lubricates the passage of the tip as it is pushed through the soil. The root cap also appears to play a role in the roots' response to gravity. Starch-containing plastids may act as statoliths, or gravity-sensing bodies.
The apical meristem of the root is just behind and surrounded on three sides by the root cap. In ferns and other lower vascular plants, there is a conspicuous apical cell in the center of the meristem. From the various faces of this cell arise files of cells that form the tissues of the root. In the gymnosperms and angiosperms, no single apical cell is present. Instead, there is a collection of cells from which the tissues arise. At the centre of this collection is a relatively inactive group of slowly dividing cells, known as the quiescent centre.
In this photo, the root was fed radioactive thymidine and an autoradiograph was made. The black dots are silver grains that indicate the incorporation of the radioactive thymidine into DNA in preparation for cell division. The quiescent center is seen to be inactive. Surrounding the quiescent center is the portion of the apical meristem which actively divides. Various layers of cells define the organization of the meristem. These can be traced from initials in the apical meristem to the mature tissues farther along the root. Thus, the epidermis is seen to arise from a tissue layer known as protoderm; the cortex, from the ground meristem; and the vascular tissue, from the procambium.
The mature root consists of several layers and tissues. The exterior is covered with epidermis, consisting of parenchyma cells lacking a cuticle. Near the end of the zone of elongation, root epidermal cells produce hairs. Not all epidermal cells develop hairs; those that do are called trichoblasts. Root hairs usually live for only a matter of days. They greatly increase the absorptive surface area of the root and are important in the uptake of minerals by the root.
Between the epidermis and the vascular cylinder at the centre of the root is the cortex. The cortex is composed of parenchyma cells which frequently store starch. Other cell types, such as sclerenchyma, may also be present. The parenchyma cells are connected through plasmodesmata, and there are usually conspicuous air spaces between the cells.
The innermost layer of the cortex is called the endodermis. This is a cylinder of tightly packed cells without intercellular spaces. The cells of the endodermis have specialised walls that contain a band known as the Casparian strip. Such bands are present in the walls perpendicular to the root surface and are composed of a mixture of compounds containing suberin and lignin. The presence of the Casparian strip makes the walls impervious to water, so that all substances entering the vascular tissue must pass through the cytoplasm of the endodermal cells.
The walls of the epidermis and the cortical cells form a continuous system through which water and dissolved substances can diffuse without entering the cytoplasm; that is, until they reach the endodermis, where the way is blocked by the Casparian strip. Here, any substance entering the vascular tissue must pass through the cytoplasm of the endodermal cells where, presumably, the flow is regulated. In young endodermal cells, only the Casparian strip is suberised, but in older roots, all walls may be suberised and thickened.
Endodermal cells opposite the phloem usually thicken first while those opposite the xylem, known as passage cells, remain thin-walled. They have that name because they are thought to allow passage of water into the xylem. The vascular tissue, or stele, occupies the centre of the root. The stele is composed of xylem and phloem, and is surrounded by the pericycle. The pericycle is the outermost layer of the stele and is directly beneath the endodermis. In young roots the pericycle is composed of thin-walled parenchyma cells, but in older roots the walls may become thickened.
The cells of the pericycle retain their meristematic capacity and are the site of formation of secondary roots. Xylem frequently forms a solid core in the centre of the stele, although in some species the centre may be composed of pith made of parenchyma cells. Xylem forms a series of ridgelike projections, extending outward from the central core to the pericycle. The phloem alternates with the ridges of xylem along the circumference of the stele. The vascular tissue differentiates from the procambium, and the direction of differentiation is from the root tip toward the more mature tissue.
The procambial cells are densely staining meristematic cells that elongate and give rise to the cells of the pericycle, xylem, and phloem. The differentiation and maturation of these tissues can be studied by tracing a file of cells from the mature portion of the root back to the procambium. No. 1 is from a mature portion, and increasing numbers proceed toward the procambium. Near the beginning of the procambium, we can see the differentiation of the protoxylem. This begins from the edges of the procambium and proceeds toward the centre. This type of development is directly opposite that seen in the stem.
Protoxylem differentiates while other cells are still elongating, and as a consequence they may be stretched and crushed. Farther up the root, and therefore later in time, the cells of the metaxylem differentiate. These cells are larger in diameter than the protoxylem and are formed after elongation is completed. They are located inside the protoxylem and may occur in the centre of the root if pith is not present. If pith is present, the metaxylem forms a ring around it. In some cases, as in onion, the first metaxylem vessel that differentiates does so in the exact centre of the root, very near the tip.
The phloem of the root consists of protophloem and metaphloem. The direction of differentiation and maturation of the phloem is the same as the xylem, namely from the younger portions of the root toward the mature and from the edges toward the centre. The phloem differentiates and matures closer to the root apex than does the xylem. The pericycle of the root is usually a single-cell-layer thick in mostangiosperms. It retains its ability to divide long after the other cells of the root have lost this ability. Because of this capacity, it is the site of formation of the secondary roots. It also functions in the thickening of the root during secondary growth, and will be discussed again in the presentation on the secondary plant body.
Before continuing our discussion of apical meristems, we should define a couple of necessary terms concerning planes of division. Divisions that occur at right angles to the surface of the meristem are said to be anticlinal, while those parallel to the surface are called periclinal.
The first sign of secondary root initiation is the periclinal division of a limited number of periderm cells. These are followed by divisions in both periclinal and anticlinal planes. These divisions result in the formation of a group of meristematic cells that soon becomes organised into an apical meristem having the same pattern as the parent root. As this new apical meristem grows, it pushes its way through the cortex and epidermis of the parent root. As the tissues of the new root differentiate and mature, the xylem and phloem develop, so that they become continuous with the xylem and phloem of the parent root.
Roots may be overlooked by the casual observer, but this is not true for the shoot. The shoot of the vascular plant consists of the leaves and stem. We will continue our examination of the primary plant body by first considering the stem and then the leaves. The stem provides the transport system that brings water and minerals from the roots to the leaves. The stem also provides the support to hold the leaves in position to receive sunlight for photosynthesis. In many plants, the stem itself also carries on photosynthesis, and in some, it is the major site of that function, as in the cacti.
Let's begin our examination of the shoot by looking at the extreme tip. Here you see a shoot tip with all of the older leaves removed to reveal a dome of cells flanked by a series of bumps of increasing size as they radiate from the dome. Each of these bumps, called leaf primordia, will develop into a leaf. We can see in this micrograph that the leaf primordia are formed close together, with little space between them. Yet, we know that in most mature plants, the leaves are not closely packed but spread out along the stem. How is this achieved?
The spacing of the leaves is achieved through the growth of the regions of the stem between the leaves. These regions are called internodes, while the places the leaves are attached are termed nodes. The extent of internodal growth has a great impact on the appearance and function of the stem. In the plant to the left, there has been considerable growth of the internodes, while in the rosette succulent to the right, there has been little.
Many gymnosperms and some angiosperms have short branches, known as short shoots or spur shoots, in which the internodes fail to grow. The contrast between the short and long shoots, in which the internodes grow normally, is striking. Here the long and short shoots of pear tree are the same age yet markedly different in length. The leaves are attached to the stem at the node, and each node may bear one, two, or several leaves. If one leaf is present at each node, the arrangement is called alternate; if two, opposite or decussate; and if several, whorled.
The arrangement of the leaves on the stem is a consequence of the order in which the leaf primordia are formed at the meristem. The arrangement of the leaves on a stem is termed the phyllotaxis, from the Greek words phyllon for "leaf" and taxis for "arrangement." The time between the formation of one leaf primordium and the next is called the plastochron, and is a highly useful measure of time in developmental studies with plants. At the nodes, the leaves join the stem at an angle known as the axil. In the axil is an embryonic shoot, consisting of an apical meristem surrounded by a group of young leaves known as a bud. All leaves have buds in their axils capable of developing into branches, although not all will do so.
A common gardening practice is to pinch off the shoot tips, which stimulates the buds farther down the stem to develop, producing a bushier plant and more flowers. In some species, under certain conditions such as over winter, the bud is covered by a number of overlapping, modified leaves known as scales. Many species lack scales and have so-called naked buds.
Buds located in the leaf or stem axil are known as axillary buds or lateral buds. Several buds, called accessory buds, may be located in a single axil. Buds located at the tip of a branch or stem are terminal buds. Floral buds may contain young flowers instead of leaves, or they may contain both young leaves and flowers, making them mixed buds. Sometimes buds arise on the stem at places other than the tip or the leaf axils: these are lateral buds.
Now let's examine the apical meristem in more detail. The apical meristem, which can be defined as the region of the shoot above the youngest leaf primordium, has three basic patterns of organisation. In ferns and the lower vascular plants, there is a large apical cell similar to that which we saw in the roots of these groups. This central cell is a large, tetrahedral cell that divides to give rise to the cells that in turn divide to form the cells of the apex.
In the gymnosperms and some angiosperms, the apical meristem shows a type of organisation known as cytohistological zonation. In this type of apical organisation, the apical cell is replaced by a group of cells known as the apical initial group. The zone below these cells, and derived from them, constitutes the central mother cells. These cells usually stain less densely, are more vacuolate, and divide less frequently than the cells surrounding them, which are known as the cells of the peripheral zone.
In most angiosperms, the apex is seen to be divided into two zones: the tunica, which consists of one or more layers of regularly arranged cells covering the surface, and the corpus, which consists of a mass of less regularly organized cells occupying the centre. Cells of the tunica divide predominantly in the anticlinal plane; that is, at right angles to the surface of the shoot. Cells of the corpus divide in all planes.
Leaf primordia in most angiosperms arise through divisions of the tunica and the immediately adjacent corpus. The leaf primordia are formed on the flanks of the apical meristem, in localised regions determined by the phyllotaxis of the plant. In species with opposite, or whorled phyllotaxis, they form simultaneously in more than one site on the apical meristem. Why primordia form where they do on the flanks of the apex has been the subject of much research because their placement is of such importance in determining the ultimate form of the plant. The most widely accepted hypothesis states that the dome of the apex, and the developing primordia, exert a physiological inhibitory field around themselves.
Where these fields are not present, a new leaf can be initiated. The cells of the shoot apex divide regularly, with those of the dome dividing more slowly than the ones on the flanks. As these dividing cells begin to enlarge, differences become evident. The surface of the shoot, the tunica in angiosperms, forms the protoderm, that in turn forms the epidermis of the plant.
Other, more deeply located cells that are densely cytoplasmic in nature, differentiate into the procambium, a meristematic tissue that in turn forms the vascular tissue. The ground tissue differentiates from the ground meristem. The procambium gives rise to the xylem, phloem, and the other vascular tissues. The differentiation of these tissues is closely tied to the development of the leaves. This diagram closely tied to the development of the leaves. This diagram shows the course of differentiation of the protophloem and the protoxylem in atypical shoot tip. Note that the three youngest leaves do not have vascular tissue associated with them, and when it does form in the fourth leaf, it is a strand of protophloem that is continuous with that of the older portion of the stem.
When the protoxylem forms, it does so in a different manner. It begins differentiation at the base of the leaf primordium and matures both toward the mature portion of the stem and up into the forming leaf. Thus, in the case of the protoxylem, differentiation and maturation occur in two directions, while in the case of the protophloem, it is in one. Metaxylem and metaphloem differentiation always progress from the mature portions of the stem into the leaves.
The protoderm, the procambium, and the ground meristem give rise to the basic tissues of the plant stem, namely the dermal, the fundamental or ground, and the vascular. Let's look at each of these in turn, starting with the dermal. The dermal tissue consists of the epidermis that forms the outer covering of the primary plant body. The epidermis is usually a single layer deep, but a multilayered epidermis is present in some species. Many stems have stomata, particularly if much photosynthesis takes place in the stem.
Hairs of various types are commonly found as part of the epidermis of the stem. Some of these are shown here on the stem tip of coleus. These may be either one-celled or multicellular, and either glandular or non-glandular. The fundamental, or ground tissue, may be present as a distinct cortex and pith, as it is in roots. However, the distinction may be far less clear, as in the case of the monocotyledons, where there is no real difference between the two. In this case, the whole tissue is called the ground tissue.
The bulk of the ground tissue is made up of parenchyma cells, although many stems have collenchyma present, either as a complete cylinder or as a ring of bundles. Sclereids may also be present. An endodermis is not usually found in stems, as it is in roots. The vascular tissue system of the stem shows considerable variation in organization. In stems, the xylem and phloem lie on the same radius, with the phloem external to the xylem.
There are three main patterns of organization based on the vascular bundle, which is the basic unit of the stem's vascular system. Vascular bundles consist of phloem, xylem, and associated cells, and run from the stem to the leaves and buds. In the first pattern, found in the gymnosperms and some angiosperms, the vascular bundles form a complete cylinder. In this case, there is a clear demarcation between the cortex and pith.
In the second pattern, found in other gymnosperms and dicotyledons, the vascular bundles develop as a cylinder of connecting strands, separated from one another by ground tissue. The ground tissue separating the strands is called interfascicular parenchyma, or pith rays.
The third pattern is found in the monocotyledons, where the vascular bundles are scattered throughout the ground tissue and no distinction can be made between the cortex and the pith. Corn and other grasses show this pattern. Most vascular bundles have the xylem on the inside and the phloem to the outside. Frequently, sclerenchyma fibres form a cap over the bundle or completely surround it. Other arrangements, however, are found, particularly in the ferns. In some ferns, the xylem completely surrounds the phloem, while in others, the reverse is true.
In some angiosperms, such as cucumber, phloem is found both outside and inside the xylem. The vascular bundles of the stem are continuous with those of the leaves. In stems where the vascular system forms a complete cylinder, the divergence of a bundle, known as a leaf trace, into a leaf, forms an opening in the cylinder, known as a leaf gap.
More than one trace may be diverted into a leaf, and a trace may diverge from the vascular system at one node, and run parallel to the vascular cylinder for one or more nodes before it enters the leaf. The vascular system of the shoot can be compared to a rope, with the vascular bundles forming tough, flexible units that give continuity and strength. The vascular system of the root, on the other hand, is much more like a rod or pipe. At some point, the rope and rod must be joined so that the plant can function as a whole. T
his place is aptly called the transition region. In the transition region, the distribution of the vascular tissue can often be highly elaborate as the two systems flow into one another. The stem is a highly versatile organ. Through the course of natural selection, a wide variety of modifications have developed in stems. We will briefly examine a few of these. Many vine stems lack internal support tissues needed to grow erect, and have developed slender, modified branches or leaves, called tendrils. These grow around various objects and bind the vine to them for support.
Stolons, or runners, are above-ground horizontal stems of relatively slender form. They grow along the surface of the soil, and at the nodes, aerial shoots and roots arise. Strawberry plants spread through the growth of stolons. Many plants have stems that remain permanently underground, and leaves grow from these to positions above ground. Such stems have a variety of names and forms, but all serve as a way to store food for growth in the next growing season. These include rhizomes, that are perennial underground stems with thickened internodes that store starch. Iris is an example.
A tuber is a greatly swollen rhizome. A good example is the potato. Corms are short, thickened, vertical stems. The "bulb" of gladiolus is a corm. A true bulb is a short stem surrounded by thickened, fleshy leaves known as bulb scales. The onion is an example of a bulb. Some thorns, as in the common hawthorn, are modified branches. The spines of cacti are modified leaves, not branches.
Let's now look at leaves. The function of leaves is photosynthesis, but as in the case of stems, there are many variations in structure and function. Some leaves, as we just saw, are storage organs, while others may be modified into spines or traps for insects. The leaves of gymnosperms are usually needle or scale-like in shape. Only a few, such as ginkgo, have flattened leaves, as found in the angiosperms.
The two great subclasses of angiosperms have markedly different types of leaves. The leaves of the dicotyledons are usually composed of three parts: the blade, which is thin and expanded; the petiole, which is a slender stemlike structure; and the leaf base, which may be extended to leaf-like stipules. Moreover, the veins in the leaf of the dicotyledons are usually characterised by a netlike arrangement. The typical monocotyledon, in contrast, has a long narrow leaf, without a petiole and with parallel veins.
Let's look at the leaves of the dicots in more detail, as the shape of the leaf is highly characteristic of the species. Leaves vary in their overall shape, the shape of the apex of the leaf, the nature of the edge or margin of the leaf, and the base of the leaf blade. The pattern of the veins in the blade may be either pinnate, with a single mid vein from which the branches diverge, like that of a pinna or feather; or palmate, with several principal veins, each with smaller branches, radiating from the bases of the blade, much as the fingers spread out from the palm of the hand.
Leaves are either simple or compound. In a simple leaf, the blade is all in one piece while in the compound leaf, the blade consists of separate, leaf-like parts, called leaflets. The arrangement of the leaflets is either pinnate or palmate. The petiole also shows various modifications. They may vary in shape and in their attachment to the leaves. In most cases, the petiole is attached to the base of the leaf, but in the case of peltate leaves, as in nasturtiums, it is attached to the lower side of the leaf. When the petiole is completely lacking, the leaf is termed sessile.
Finally, the leaf bases and stipules show a range of variation. The base may extend around the stem or be much reduced in size. The stipules may be large and leaf like or so small as to be difficult to see. Leaves of the monocotyledons show somewhat less variation, but there is still a great range in size and shape. The grass leaf, however, is typical of the leaf of many monocot species. It is divided into two parts: the blade is the expanded part of the leaf, and the sheath is the part of the leaf below the blade that curves around the stem. The sheath is usually green and may be as long as the blade. It completely encircles the stem and may extend from one node to the next.
Many tropical monocotyledons diverge strikingly from this pattern. Blades maybe broad and may not show parallel venation. In the banana, a tropical monocot, the huge leaves have large, overlapping leaf bases, which actually form the trunk of the banana tree. The stem itself is short and inconspicuous. Internally, leaves consist of three tissue systems: epidermis, mesophyll, and veins, or vascular tissue. The leaf has an upper and lower epidermis, often with a variety of hairs, both simple and complex, and with stomata and guard cells. Some species have stomata only on the lower surface; others have them on both surfaces. Many monocotyledons and conifers have them arranged in rows.
The mesophyll lies between the upper and lower epidermis. In grasses, the mesophyll cells are usually the same throughout, but in dicots the mesophyll has two layers: the palisade parenchyma, composed of elongate cells, and the spongy parenchyma, composed of irregularly shaped cells. There is considerable space between all of the mesophyll cells, and it is important for the gas exchange necessary for photosynthesis.
Palisade parenchyma have more chloroplasts than spongy cells do, and the palisade may be several times thicker than the spongy. The entire mesophyll is penetrated by veins of xylem and phloem, so that virtually every mesophyll cell is only a few cells away from a vein. In a vein, the xylem is arranged on the top toward the upper epidermis, while the phloem is on the bottom. In many monocots and some dicots, the vein is surrounded by a special layer of parenchyma cells known as the bundle sheath.
The bundle sheath cells often completely surround the veins and veinlets to the very end, thus separating the vein from the mesophyll. The presence of the bundle sheath has been related to the type of photosynthetic pathway found in the plants. Collenchyma and sclerenchyma cells may also be found in the leaf associated with the vein. Such tissue adds strength to the vein and the leaf.
We have covered the primary plant body, those parts that are formed as a result of the activities of the apical meristems. Many plants have extensive growth that is the product of the lateral or secondary meristems. This growth is of great importance in many woody plants and is treated in our next presentation, "The Secondary Plant Body."