Anatomy of Plants

Contents

Characteristics and Structures of Plants

Plant Characteristics:


• Photosynthesis - Plants obtain their energy by converting light energy into chemical energy by means of photosynthesis, which involves two basic sets of reactions that take place in the chloroplast. The light dependent reactions involve the absorption of light energy by specialized pigments including chlorophyll, and the reaction within the thylakoid. The energy is absorbed by electrons and used to create ATP and a compound called NADPH provides

		the hydrogen for the subsequent reaction. These compounds carry energy and hydrogen ions (in the case of NADPH) into the light independent reactions. These reactions bring carbon dioxide into the plant's metabolism. Eventually, all these materials are combined into glucose and other carbohydrates (see Figure 01). In general, the plant uses its root to extract water in the soil, its leaf to absorb carbon dioxide from the air,
Figure 01 Photosynthesis [view large image]	Figure 02 Plant Structures [view large image]	energy from the sun, and the stem for transportation of water and nutrients (see Figure 02). Oxygen is a by product exits through the leaf.

Leaves are the organs of photosynthesis in vascular plants. A leaf usually consists of a flattened blade and a petiole, which connects the blade to the stem. The blade may be single or it may be composed of several leaflets. Externally, it is possible to see the pattern of the leaf veins that bring water to the leaf; they are the final extensions of the vascular tissue. The cross section of a typical leaf, its external and internal structures are shown in Figure 03a. At the top and bottom of the leaf is a layer of epidermal tissue that often bears protective hairs and glands that produce irritating substances to discourage herbivores. The epidermis is covered by a waxy cuticle that keeps the leaf from drying out. Unfortunately, it also prevents gas exchange because the cuticle is not gas permeable. However, the epidermis, particularly the lower one, contains openings called stomata that allow gases to move into and out of the leaf. Each stoma has two guard cells that regulate its opening and closing. The body of a leaf is composed of mesophyll tissue, which contain many

		chloroplasts and carry on most of the photo-synthesis for the plant. Leaf veins consist of a strand of xylem and a strand of phloem surrounded by a bundle sheath. The bundle sheath cells in C3 plants do not contain chloro-plasts; while the C4 plants characteristically have chloroplasts and are more efficient in fixing carbon dioxide. Figure 03b shows a leaf
Figure 03a Leaf Anatomy [view large image]	Figure 03b Leaf Sample [view large image]	sample magnified 420X. The mesophyll can be divided into two parts - the palisade cells contain chloroplasts, while the spongy cells are more involved with gas exchange.




Locomotion - It is often said that since plants simply transform light into chemical energy, they need only to be anchored firmly in one place with a maximum surface area to capture sunlight. There is no need for plants to move or to have sophisticated nervous systems. Actually, some of them have quite elaborate and creative ways of moving their leaves in response to a wide variety of stimuli, such as touch and light. Leaf movements can be as fast as a millisecond in the Venus flytrap (Figure 04) or as slow as a half-hour in sun-tracking plants. Motor cells located in the region—called the pulvinus—where the leaf connects to the stem, control the movement. These cells either shrink or swell because of the influx (inward flow) or efflux (outward flow) of water. For example, if the cells at the top of the pulvinus swell and the bottom cells shrink, the leaf tilts downward. The changes in cell volume are a direct result of changes in the concentration of certain ions in the cell, such as potassium and chloride. For example, when a large amount of potassium enters the cell through special potassium channels, a large amount of water must enter so that the concentration of potassium stays constant. It is unclear what is the signaling mechanism to open the ion channels. Different plants have evolved different signaling mechanisms depending on what triggers the leaves to move.

Figure 04 Venus Flytrap [view large image]

• Reproduction and Growth - Asexual reproduction is very common among plants, while sexuality is not well-defined (90% have both male and female parts). Plant cells are rarely terminally differentiated. Since most cells are totipotent, detached limb can be re-generated readily. Plants do not establish a germ line (special cells destined to produce gametes). Higher plants have sex organs with an outer layer of nonreproductive cells that can prevent desiccation of gametes - the flower. The developing diploid embryo is protected from drying out by providing it with water and

nutrients within the female reproductive structure - the fruit. Growth in plants can be indeterminate. All plants have a two-generation life cycle known as alternation of generations. This means that a plant exists in 2 forms: the haploid generation is the gametophyte that produces gametes; and the diploid generation is the sporophyte that produces spores by meiosis. Spores are haploid structures that develop or mature into the gametophyte plant. Usually, lower plants spend their life longer in haploid (1N) generation (see Figure 05).

Figure 05 Life Cycle of Plants [view large image]

• In addition to a cell membrane, plant cells are surrounded by a cell wall (Figure 06) that varies in thickness depending on the function of the cell. All plant cells have a primary cell wall, having as its main constituent cellulose molecules united into threadlike microfibrils. Several microfibrils, in turn, are found in fibrils, and within the wall there are layers of fibrils lying at right angles to one another for added strength (Figure 06). Some cells in woody plants have a secondary cell wall that forms inside the primary cell wall. Secondary cells contain lignin (major component of wood), a substance

  	that makes secondary cell walls even stronger than primary cell walls. Plant support cells have secondary walls; the cell dies and the strong wall remains as supporting material. The plasmodesmata in Figure 06 is a very fine thread of cytoplasm that passes through openings in the walls of adjacent cells and forms a living bridge between them.
  Figure 06 Plant Cell Wall [view large image]

The Root System: It is usually underground. It anchors the plant in the soil. The root absorbs then conducts water and minerals to the stem. It is a food storage. The Shoot System: It is usually above ground to elevates the plant from the soil. The shoot system includes the stem, the leaves, and the reproductive organs. It provides many functions including - photosynthesis. reproduction and dispersal. nutrients and water conduction. The internal structures and specific functions are shown in Figure 02.

Figure 07 Plant Anatomy [view large image]

Algae and Classification of Plants

		plants (see "Evolution of Micro-organisms and Plants" in the appendix) because both of these groups possess chlorophylls a and b, both store reserve food as starch, and both have cell walls that contain cellulose (Figure 08). Seaweeds such as Ulva are multicellular algae carrying chlorophyll. They are anchored firmly to the rock by holdfasts. Their appearance give a false impression that they are plants having root, stem, and leaf (Figure 09).
Figure 08 Green Alga [view large image]	Figure 09 Seaweed [view large image]

While a few of the algae such as some species of Fucus follow the diplontic lifecycle, most of them such as the green algae Chlamydomonas spend most of their life in the haploid generation. Usually, this protist practices asexual reproduction, and the adult divides to give zoospores that resemble the parent cell. During sexual reproduction, gametes of two different strains come into contact and join to form a zygote. A heavy wall forms around the zygote, and it becomes a zygospore. The zygospore is able to survive until conditions are favorable for germination and subsequent production of 4 zoospores by meiosis. The gametes shown in Figure 10 are isogametes; that is, they look exactly alike. Sexual reproduction aids the process of evolution because it offers means to produce variations in addition to mutations. Note that embryo is absent in this kind of lifecycle.

Figure 10 Green Algae Lifecycle [view large image]

Table 01 Classification of Plants

Bryophytes (Moss, Liverwort)

The bryophytes include liverworts and mosses. Most species of liverworts are "leafy" and look somewhat like mosses, but close examination shows that the body of a liverwort has distinct top and bottom surfaces, with numerous rhizoids (rootlike hairs) projecting into the soil. In contrast, a moss has a stemlike structure with radially arranged, leaflike structures. Rhizoids anchor the plant and absorb minerals and water from the soil. Because bryophytes do not have vascular tissue, they lack true roots, stems, and leaves. Instead, they have rhizoids, stemlike and leaflike structures (Figure 11).

Figure 11 Bryophytes [view large image]

Antheridia and archegonia are both multicellular structures, and each has an outer layer of jacket cells that protects the enclosed gametes from drying out. After an egg is fertilized, the developing sporophyte is retained within the archegonium as an embryo. The sporophyte, which is dependent on the gametophyte, consists of a foot that grows down into the gametophyte tissue, a stalk (seta), and an upper capsule, or sporangium, where meiosis occurs and where haploid spores are produced. In some species of mosses, a hoodlike covering is carried upward by the growing sporophyte. When this covering and the capsule lid falloff, the spores are mature and ready to escape. The rings of "teeth" projected inward from the edge of the capsule allows spores to be released only at times when the weather is dry (when they are most likely to be dispersed by wind). When a spore lands on an appropriate site, it germinates. The single row of cells that first appears branches, giving an algalike sturcture called a protonoma. After about three days of favorable growing conditions, new moss plants appear at intervals along the protonema. Each of these consists of the rootlike rhizoids and the upright shoots of a moss gametophyte. The gametophytes produce gametes, and the moss life cycle begins again.

Figure 12 Moss Life Cycle [view large image]

Sperms are released when the antheridium ruptures, thus allowing them to swim freely in a water film toward the archegonium. The zygote is the first cell of the new sporophyte just after fertilization. The zygote divides by mitosis into a multicellular embryo within the archegonium (see Figure 12 and 13). This is the crucial step that separates plants from algae. The embryo then inserts an absorbing organ called the foot into the female stem tip. The other end of the embryo grows up and above the female stem to form a stalk (seta) and sporangium (capsule) anchored in the old archegonium. Early in its development the sporophyte is typically green, but by the time it is mature it is usually non-photosynthetic and dependent on the gametophyte for water and nutrients. Within the sporangium special cells called sporocytes divide by meiosis to produce thick-walled haploid spores. In the more advanced plant species, the embryo is enclosed within the seed as shown in Figure 21.

Figure 13 Moss Embryo [view large image]

Pteridophytes (Fern, Club Moss, Horsetail)

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 [view large image]

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 [view large image]

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 [view large image]

		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 [view large image]

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.

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 [view large image]

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

Gymnosperms (Cyced, Ginko, Conifers)

The gymnosperms produce naked seeds; that is, the seeds are not enclosed by fruit. There are four divisions of gymnosperms (Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta; see Figure 19). Cycads are cone-bearing, palmlike plants found today mainly in tropical and subtropical regions. Only one species of ginkgo, the maidenhair tree, survives today. The gnetophyta has only three genera left. The largest group of gymnosperms is the cone-bearing conifers, which include pine, cedar, spruce, fir, and redwood trees. These trees have needlelike leaves that are well adapted to not only hot summers but also cold winters and high winds. Most gymnosperms are evergreen trees.

Figure 19 Gymnosperms [view large image]

opening. During pollination, pollen grains are transferred from the male cone to the female cone. Once enclosed within the female cone, the pollen grain develops a pollen tube that slowly grows toward the ovule. The pollen tube discharges two nonflagellated sperms. Only one of the sperms fertilizes an egg in the ovule 15 months after pollination. After fertilization, the ovule matures and becomes the seed composed of the embryo, its stored food, and a seed coat. Finally, in the third season, the female cone, by now woody and hard, opens to release its seeds, whose wings are formed from a thin, membranous layer of the cone scale. When a seed germinates, the sporophyte embryo develops into a new pine tree, and the cycle is complete (Figure 20).

Figure 20 Conifer Life Cycle [view large image]

including small mammals, birds, and insects. The embryo is found in the corrosion cavity, a pit in the centre of the megagametophyte that is fully filled by the embryo in mature seeds. It consists of the cotyledons (first leaf), shoot apical meristem, root apical meristem, root cap and suspensor. The cotyledons and shoot apical meristem point towards the wider end of the seed; while the radicale (embryonic root) and suspensor are at the more pointed end. The suspensor is found at the base of the root cap and plays a role early in embryo development by pushing the embryo into the megagametophyte.

Figure 21 Seed Anatomy [view large image]

Angiosperms (Oak, Maple, Basil, ...)

leave evergreen trees (e.g., boxwood, ...) of the tropical zone, are angiosperms, although sometimes the flowers are inconspicuous. All herbaceous (nonwoody, nonpersistent) plants (e.g., basil, ...) common to our everyday experience, such as grasses and most garden plants are flowering plants (Figure 22). Angiosperms are adapted to every type of habitat, including water (e.g., water lilies, ...). Angiosperms have well-developed vascular and supporting tissues. Their xylem tissue contains vessel elements beside the tracheids. Thus the woody angiosperms are considered as hardwood trees, whereas the gymnosperms are softwood trees.

Figure 22 Angiosperms [view large image]

The angiosperms are divided into two classes: the monocots (e.g., rice, ...) and the dicots (e.g., potato, ...). The distinction between these two groups is not always clear, some of the general characteristics (including the gymnosperms) are outlined in Table 02 below.

Table 02 Monocots, Dicots, and Gymnosperms Comparison

apple, tomato, peach, ...) and some of which are dry (e.g., pea enclosed by pod, nut, grain, ...). They all provide protection for the seeds. The life cycle of the flowering plant is shown in Figure 23. Within a flower, there is a diploid megaspore mother cell in each ovule of the ovary. The mother cell undergoes meiosis, producing one functional megaspore, whose nucleus divides mitotically until there are eight haploid nuclei. This is the female gemetophyte, which sometimes is called the embryo sac. At one end of the embryo sac there the three cells, one of which is the egg cell. Male gametophytes are produced in the stamens. An anther contains four pollen sacs with many microspore mother cells, each of which undergoes meiosis to four microspores. After a mitotic division, each misrospore has two cells, one of which later divides again to give two sperm. Pollination, which is simply the transfer of pollen from the anther to the stigma, is brought about by wind or with the assistance of a particular pollinator. The plant uses the pollinator to ensure cross-pollination, and the pollinator uses the plant as a source of food in the form of nectar. When a pollen grain lands on a stigma of the same species, it germinates, forming a pollen tube. The pollen tube grows as it passes between the cells of the stigma and the style to reach the female gemetophyte.

Figure 23 Flowering Plant Life Cycle [view large image]

Double fertilization takes place to produce seeds and fruits as shown in Figures 22 and 23. One sperm nucleus from the pollen tube unites with the egg nucleus, forming a zygote, and the other sperm nucleus unites with the polar nuclei, forming a triploid (3N) endosperm nucleus. The endosperm nucleus divides, forming the endosperm, which is a nutrient material for the developing embryo and sometimes for the young seedling as well. The zygote develops into an embryo. The outer layers (integuments) of the ovule harden and become the seed coat. A seed is a structure formed by the maturation of the ovule; it

Figure 24 Seed and Fruit [view large image]

contains a sporophyte embryo plus stored food. The ovary and sometimes other floral parts develop into the fruit. A fruit is a mature ovary that usually contains seeds. Therefore, angiosperms are said to have covered seeds.