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]

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