Anatomy of Plants

Pteridophytes (Fern, Club Moss, Horsetail)

Vascular plants (also called tracheophytes) are believed to have evolved sometime during the late Silurian Period. The primitive vascular plants include the whisk ferns (Psilopsid), the club mosses, and the horsetails (see Figure 14). The whisk ferns is of particular interest because they may be the most primitive (see Figure 05). It bears considerable resemblance to

the extinct rhyniophytes. Its sporophyte consists of stems with scalelike structures but no leaves. There is a horizontal stem (lacking roots), from which rhizoids grow, and there are green, photosynthetic, upright branches with tiny, scalelike structures that grow upward. Sporangia are located on the branches. The gametophyte is separate from and smaller than the sporophyte; it also lacks vascular tissue. In general, the tracheophytes have two types of vascular tissue. Xylem conducts water and minerals up from the soil, and phloem transports organic nutrients from one part of the body to another. Because they have vascular tissue, the specialized body parts of tracheophytes can be called properly roots, stems and leaves.

Figure 14 Ferns
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Figure 15 shows the life cycle of a common fern of the temperate zone. Young fronds grow in a curled-up form called fiddleheads, which unroll as they grow. The fronds often are subdivided into a large number of leaflets. The sporophyte fern plant represents the dominant generation. Sporangia develop in clusters called sori, which are protected by a covering, the indusium (not shown). Within the sporangia, meiosis occurs and spores and produced. The gametophyte is a tiny (1-2 cm),

heart-shaped structure called a prothallus. The antheridia and archegonia develop on the under side of a prothallus. Fertilization takes place when moisture is present because the spiral-shaped sperm must swim from the antheridia to the archegonia. The resulting zygote soon develops into a sporophyte embryo consisting of a foot, a root, a stem, and a leaf. The root grows down into the soil, and the frond grows upward through the prothallus notch. As the sporophyte matures, the prothallus shrivels and disappears. Since the gametophyte lacks vascular tissue, and the swimming sperm relies on moisture to approach the egg, ferns are likely to be found in habitats that are at least seasonally moist. Once established, the sporophyte of some ferns can spread by vegetative reproduction into drier areas because this generation has vascular tissue.

Figure 15 Fern Life Cycle
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The vascular structures in the ferns are primitive in comparison to the more advanced plant species. They have the rhizome, which can be compared to the stem of a flowering plant. In many cases the rhizome can be inconspicuous or even entirely

underground. Rhizomes of tree ferns on the other hand may be 60 cm in diameter and up to 12 meters tall. The fronds (leaves) arise from the upper side or in one or more rows laterally on each side from the rhyzome. They are composed of two main structures: the stipe (stalk) and the blade (the leafy outcroppings). Roots are formed from the rhizomes or sometimes from the stipe. The roots usually do not divide once they grow from the rhizome. Tree fern roots grow down from the crown and help thicken and strengthen the trunk (Figure 16). The roots anchor the plant to the ground and absorb water and minerals. The internal structures of the rhizome, the root, and the leaf are shown in Figure 16.

Figure 16 Fern Anatomy
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In the more advanced plant species, the outermost tissue of the stem is the epidermis. The stem has distinctive vascular bundles, where xylem and phloem are found. In each bundle, xylem is typically found toward the inside and phloem is toward the outside. In the dicot stem, the bundles are arranged in a distinct ring that separates the cortex from the central pith (see Figure 02, and 17). The cortex is sometimes green and carries on photosynthesis, and the pith may function as a storage site for the products of photosynthesis. In the monocot stem, the vascular bundles are scattered throughout the stem, and there is no well-defined pith. It is similar to the more primitive type shown in Figure 16. Secondary growth of stems is seen primarily in woody plants, such as trees that live for many years. Almost all trees are dicots. Primary growth in woody plants occurs for a short distance beneath the apical meristem. Secondary growth occurs in the vascular and cork cambia (see Figure 02, and 17). Vascular cambium begins as meristematic cells between the xylem and the phloem of each vascular bundle. Then these cells join to form a ring of meristematic tissue adding to the girth of the stem. Cork cambium is located beneath the epidermis. It produces tissue that disrupts and replaces the epidermis with cork cells, which are impregnated with suberin (a waterproof substance). Dead cork allows gas exchange in pockets of loosely arranged cells, called lenticels. A woody stem has three

		distinct areas: the bark (containing cork, cork cambium, cortex, and phloem), the wood, and the pith. In large trees, only the more recently formed layer of xylem, the sapwood, functions in water transport. The older inner part, called the heartwood, becomes plugged with deposits, such as resins, gums, and other substances. Figure 17a provides a more detailed illustration with the structures of a young woody stem. Figure 17b shows
Figure 17a Stem Anatomy [view large image]	Figure 17b Stem Sample [view large image]	the cross-section through the stem of a Geranium plant.

Figure 18a depicts a longitudinal section of a root. At the bottom is an area of cells called the root cap, a thimble-shaped mass of parenchymal cells (relatively unspecialized cells) that is a protective covering for the root tip, and the cells in the next region – the region of cell division. Cells in the root cap have to be replaced constantly because they are ground off as the root pushes through abrasive soil particles. The next area - the zone of cell division - is the area where new cells are continually being formed through repeated cell divisions. These cells are thin-walled and easily ruptured by soil particles were it not for the root cap's protection. Next is the zone of cell elongation. Here the cells take up large amounts of water and increase in volume. The increase in cell volume of these cells is primarily responsible for pushing the root through the soil. The next zone is the zone of cell maturation and differentiation. The fully elongated cells in these zones matured and began differentiating into various tissues such as the xylem, phloem, pith, cortex, and others. This zone, the zone of maturation and differentiation begins where the root hairs first become

Figure 18a Root Anatomy
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evident. These root hairs are only extensions of the epidermal cells - as may be seen in the inset drawing on the left of the figure. Branch roots have formed beyond these zones.

The absorbed water and minerals pass through the cortex, a tissue composed of parenchymal cells. The water and minerals are forced by a strip of waxy material (the Casparian strip) in the endodermis to move one way into the vascular cylinder. Within the vascular cylinder, water and minerals are transported upward by way of the xylem and the products of photo-

synthesis most often are transported downward by way of the phloem for storage in the cortex. Lying between the endodermis and the vascular tissue is the pericycle, composed of parenchymal cells, that retains the ability to undergo cell division and on occasion produces branch roots. The pericycle alos contributes to the formation of vascular cambium, which is meristematic tissue lying between xylem and phloem that is capable of producing new vascular tissue. Monocot roots often have pith, which is centrally located ground tissue. In a monocot root, pith is surrounded by a ring of alternating xylem and phloem bundles. They also have pericycle, endodermis, cortex, and epidermis.

Figure 18b Root of Iris
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Figure 18b shows the typical cross-section of monocotyledonous plants such as the Iris.