THE PRIMARY PLANT BODY

1. The Primary Plant Body
2. 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 meristem near the ends of the root and shoot. There the cells divide, elongate, and differentiate forming tissues and tissue systems.
3. That part of the plant formed as a result 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. Secondary growth will be covered in another module.
4. In this module we will examine the primary tissues and tissue systems present in the root, stem, and leaves of the primary vegetative body. Let’s start with the root.
5. 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.
6. Most dictoyledons and gymnosperms have a root system characterized by a large dominant vertical root known as a taproot. Grasses and many other monocotyledons, as well as many dicots, 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.
7. The root system of any given plant is largely an adaptation to that plants 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 rain fall. Many cacti, on the other hand, have extremely shallow but widespread roots which soak up as much water as possible from the infrequent rain.
8. 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.
9. The growth in length of a root occurs thru the division and elongation of cells located in the apical portion of the root.
10. The extreme tip of the root is covered by a mass of cell called the root cap. The cells of the root cap are parenchyma (note from Amy; see lab manual for definition), 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 thru 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.
11. 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.
12. 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 center of this collection is a relatively inactive group of slowly dividing cells, known as the quiescent center. In this photo the root was fed radioactive thymidine and an autoradiograph was made. The black dots are sliver grains that indicate the incorporation of the radioactive thymidine into DNA in preparation for cell division. The quiescent center seems to be inactive. (Note from Amy; it's a picture of a root tip, with dividing nuclei stained. Therefore, all the cells surrounding the quiescent center are dark; the q.c. itself is unstained.)
13. 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.
14. 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.
15. Between the epidermis and the vascular cylinder at the center of the root is the cortex. The core is composed of parenchyma cells, which frequently store starch. Other cells types, such as sclerenchyma, may also be present. The parenchyma cells are connected thru plasmodesmata and there are usually conspicuous air spaces between the cells.
16. 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 specialized walls that contain a band known as the Casparian Strip. Such bands are present in the walls perpendicular to the root surface and are compose of a mixture of compounds containing suberin and lignin. (note from Amy; this structure serves as a dam that blocks the passage of materials and organisms into the stele of the root. It's like a seal that runs the length of the mature root. The best analogy I have is the plastic bag over a newspaper; in this case, the newspaper is the stele.)
17. The presence of the casparian strip makes the walls impervious to water so that all substances entering the vascular tissue must pass thru the cytoplasm of the endodermal cells. The walls of the epidermis and the cortical cells form a continuous system thru 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 thru the cytoplasm of the endodermal cells where, presumably, the flow is regulated.
18. In young endodermal cells only the casparian strip is suberized but in older roots all walls may be suberized and thickened. Endodermal cells opposite the phloem usually thicken first while those opposite the xylem, known as passage cells, remain thin walled (note from Amy; take a look at a diagram of how a root is arranged. The xylem and phloem alternate around the edge of the stele. See figures 9.8 and 9.13 of your atlas. In figure 9.14, the line for #8 ends on a suberized cell of a casparian strip). They have that name because they are thought to allow passage of water into the xylem (in 9.14, you can see the alternation of casparian strips and passage cells quite clearly).
19. The vascular tissue or STELE occupies the center 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.
20. The xylem frequently forms a solid core in the center of the stele, although in some species the center may be composed of PITH made up of parenchyma cells.
21. The 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.
22. The vascular tissue differentiates from the procambium and the direction of differentiation is from the root tip toward the more mature tissue (note from Amy; this means that differentiation begins near the root tip and ends when the cells are mature). The procambial cells are densely staining meristematic cells that elongate and give rise to the cells of the pericycle, xylem, and phloem (the stele). 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. Number one is from a mature portion and increasing numbers proceed towards the procambium.
23. This approach was used to produce the diagram shown here. 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 center (note from Amy: this means the procambium cells near the edge of the stele turn into xylem first, while the cells near the center turn into xylem last.) This type of development is directly opposite that seen in the stem. Protoxylem differentiates while other cells are still elongating and as a consequence it may be stretched and crushed . (note from Amy--this is the "temporary plumbing" that gets replaced by much better xylem as the root matures.)
24. 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 complete. The are located inside the protoxylem and may occur in the center of the root, if pith is not present. (From Amy; basically, the crappy xylem is close to the exterior of the stele and gets crushed and destroyed when the better metaxylem forms later on and closer to the center of the stele.) 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 center of the root very near the tip (and is still surrounded by that crappy protoxylem….).
25. The phloem of the root consists of PROTOPHLOEM and METAPHLOEM. The direction of differentiation and maturation is the same as the xylem, namely from the younger portions of the root toward the mature, and from the edges toward the center (note from Amy--same song, different verse. The crappy stuff is toward the exterior of the stele, and gets replaced by the good stuff as soon as the root can make it.). The phloem differentiates and matures closer to the root apex than does the xylem.
26. The pericycle of the root is usually a single cell-layer thick in most angiosperms. 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 discusses again in the module on the secondary plant body.
27. 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.
28. The first sign of a secondary root initiation is the periclinal division of a number of periderm cells (note from Amy--they mean periCYCLE 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 organized into an apical meristem having the same pattern as the parent root.
29. As this new apical meristem grows, it pushes its way thru 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 (note from Amy; all the plumbing connects.).
30. Roots may be overlooked by the casual observer (note from Amy; until you trip over them), 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.
31. 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.
32. 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 the 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?
33. The spacing of the leaves is achieved thru the growth of the regions of the stem between the leaves. These regions are called INTERNODES, while the places the leaves are attached are called 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.
34. 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.
35. 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 (note from Amy; decussate??? Only in this film, I think!), 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.
36. 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 primordia and the next is called the plastochron and is a highly useful measure of time in the developmental studies with plants.
37. 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.
38. In some species under certain conditions such as over winter, the bud is covered, by a number of overlapping modified leaves known as SCALES. Such a winter bud is shown here. Many species lack scales and have so-called NAKED BUDS.
39. 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 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.
40. 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 organization. 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.
41. In the gymnosperms and some angiosperms, the apical meristem shows a type of organization known as CYTOHISTOLOGICAL ZONATION. In this type of apical organization 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.
42. 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 center. The cells of the tunica divide predominantly in the anticlinal plane, that is, at right angles to the surface of the shoot. The cells of the corpus divide in all planes.
43. Leaf primordia in most angiosperms arise thru divisions of the tunica and the immediately adjacent corpus. The leaf primordia are formed on the flanks of the apical meristem in localized 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.