Plant Organs PDF

I- EXTERNAL FORM OF PLANT ORGANS Dr. Sanaa Abdel Rahman Mostafa Zaghlool Agric. Botany Department [email protected]  Roots Root systems Roots function...

I- EXTERNAL FORM OF PLANT ORGANS Dr. Sanaa Abdel Rahman Mostafa Zaghlool Agric. Botany Department [email protected]  Roots Root systems Roots function Taproot fibrous root Absorption of water Anchorage and minerals Most dicot plants Monocot ❑ Taproot develops from the radicle. ❑ Fibrous root system develops from adventitious roots. ❑ Adventitious refers to structures arising at unusual places, such as roots growing from stems  Root systems A. A fibrous root system of a grass. B. A taproot system Four regions or zones have recognized in developing young roots ❑ The root cap: the root cap is composed of a thimble-shaped mass of parenchyma cells covering the tip of each root. Root cap protect from damage the delicate tissues behind it as the young root tip pushes through soil particles. The root cap has another function, the perception of gravity with the help of amyloplasts. ❑ The region of cell division: the region of cell division is in the center of the root tip and surrounded by the root cap. It is composed of an apical meristem. ❑ The region of elongation: the cells become several times their original length and somewhat wider while the tiny vacuoles merge and grow until one or two large vacuoles, occupying up to 90% or more of the volume of each cell. ❑The region of maturation: most of the cells mature into the various distinctive cell types of the primary tissues in this region, which is sometimes called the region of differentiation, or root-hair zone. A longitudinal section through a dicot root tip. A. Regions of the root. B. Locations of the primary meristems of the root. Root structure A. A radish (Raphanus) seedling shortly after germination, showing the root hair zone. B. A diagram of an enlargement of a longitudinal section of a small portion of a root- hair zone, showing root hairs in contact with soil particles.  SPECIALIZED ROOTS ❑ Food-storage roots: the roots are enlarged and store large quantities of starch and other carbohydrates. ❑ Propagative roots: Many plants produce buds in unusual places, or adventitious buds, along the roots that grow near the surface of the ground. The buds develop into aerial stem called sucker. ❑ Pneumatophores: Some swamp plants such as the black mangrove, develop special “spongy” roots called pneumatophores, which above the surface and facilitate gas exchange between the atmosphere and the surface roots to which they are connected.  SPECIALIZED ROOTS Food-storage roots Propagative roots ❑Aerial roots: ▪ Velamen roots of orchids have an epidermis several cells thick aids in the absorption of rainwater, but some now suspect it may function more in preventing loss of moisture from root. ▪ Prop roots of corn produced toward the base of the stems, support the plants in high wind. ▪ Prop roots of tropical figs or banyan trees produced roots that grow down from the branches until they contact the soil. Once they are established, they continue secondary growth and look just like additional trunks. ▪ Adventitious roots of ivies ▪ Photosynthetic roots of certain orchids and corn The aerial (velamen) roots A. Pneumatophores (foreground) of tropical mangroves of orchids B. Close-up photograph of pneumatophores. A. Prop roots of corn. B. A banyan (Ficus) tree with many large prop roots ❑ Contractile roots: pull bulbs and corms little deeper into the soil each year as additional sets of contractile roots are developed. The bulbs continue to be pulled down until an area of relatively stable temperature is reached. ❑ Buttress Roots: Some tropical trees growing in shallow soils produce huge, buttress- like roots toward the base of the trunk, giving them great stability. ❑ Parasitic roots: A number of plants, such as the dodders and broomrapes have no chlorophyll (necessary for photosynthesis) and have become dependent on plants with chlorophyll for their nutrition. ❑ Root Nodules: numerous small swellings can be seen along the roots of peas and beans (and other members of the Legume Family). These root nodules contain bacteria (nitrogen-fixing bacteria) that supplement the plant’s nitrogen supply. Contractile roots Buttress roots Root nodules A. Pale stems of a parasitic plant (dodder— Cuscuta) twining about other vegetation. B. A close-up view of dodder.  Stems Leaves arrangement Alternately Opposite Whorled (Spiral) ❑ Axillary bud: the bud located in the angle between a petiole and the stem Axillary buds may become branches, or they contain tissues that will develop into the next season flower ❑ Winter Bud (Covered):buds are protected by one to several bud scales, which fall off when bud tissue growth begins ❑ A terminal bud: is often present at the tip if each twig. It usually resembles an axillary bud, but it is frequently a little larger. normally produce tissues that extend the length of the twig during growing season. ❑ Stipule: The scars at the base of the petiole.  woody twig A. The twig in its winter condition. B. The twig as it appeared the summer before.  SPECIALIZED STEMS ❑ Specialized stems: stems that perform specialized functions; these stem are modified accordingly. ❑ Rhizomes: horizontal stems that grow below ground, often near the surface of the soil. Adventitious roots produced all along the rhizome, mainly on the lower surface. ❑ Stolons: more or less horizontal stems that are also called runner, differ from rhizomes in growing above ground, generally along the surface; they also have longer internodes. ❑ Tubers: As food accumulates at the tips of underground stolons, several internodes swell, becoming tubers. The “eyes” of the potato tuber are actually nodes formed in spiral around the modified stem. ❑ Bulbs: they are actually large buds with a small stem at the lower end surrounded by numerous fleshy leave. Adventitious roots grow from the bottom of the stem, but the fleshy leaves comprise the bulk of the bulb tissue, which function in food storage. ❑ Corms: they superficially resemble bulbs, differ from them in being composed almost entirely of stem tissue except for the few papery scalelike leaves sparsely covering the outside. Adventitious roots are produced at the base. ❑ Climbing plants: ▪ Rambles: stems simply rest on the tops of other plants, ▪ Tendrils: these are specialized stems in grape but are modified leaf or leaf parts in the peas. ▪ In English ivy, the stem climb with the aid of adventitious roots. Stems modified for climbing  Cladophylls In the butcher-broom plant, stems are flattened and very leaflike in appearance. In the center of each cladophyll is a node bearing very small scalelike leaves with axillary buds. The feathery appearance of asparagus is due to numerous small cladophylls.  Leaves ❑ All leaves originate as primordial in the buds. ❑ At maturity, most leaves consist of stalk, called the petiole, and a flattened blade, or lamina, which have a net work of veins (vascular bundles). ❑ A pair of appendages, called stipules, are sometimes present at the base of the petiole. ❑ Occasionally, leaves may lack petioles; when they do, they are said to be sessile. Leaf functions: ▪ Green leaves capture the light energy available to them by means of the most important process for life on earth. This process called, photosynthesis. ▪ Leaves also perform other functions. For example, all living cells respire, and in process of this and other metabolic activities, waste products are produced. These wastes are deposited outside the plant when the leaves are shed. ▪ Leaves are also involved in the movement of water absorbed by the roots and transported throughout the plant. ▪ Most of water reaching the leaves evaporates into the atmosphere by the process known as transpiration.  Types of leaves Simple with an undivided blade Compound with blade divided into leaflets in various ways Pinnately compound leaves the leaflets in pairs along a central stalk-like rachis palmately compound leaves all the leaflets attached at same point at the end of the petiole bipinnately compound leaf the leaflets of pinnately compound leaf may be subdivided into still smaller leaflets The arrangement of veins in a leaf or leaflet blade: ▪ Pinnately veined leaves: there is main vein, called midrib with secondary veins branching from it. ▪ Palmately veined leaves: several veins fan out from the base of the blade. The larger veins are parallel to one another in monocots and diverge from one another in various ways in dicots. ▪ Dichotomous venation: no midrib or other large veins are present. Instead, the veins fork evenly and progressively from the base of the blade to the opposite margin.  Parallel venation Dichotomous venation A bipinnately compound leaf. Venation  SPECIALIZED LEAVES ❑ Tendrils: leaves are partly or completely modified as tendrils. These leaves when curled tightly around more rigid objects, help the plant climbing or in supporting weak stems. ❑ Reproductive leaves: The succulent leaves of air plants have little notches along the leaf margins in which tiny plantlets are produced, complete with roots and leaves. ❑ Floral leaves (Bracts): specialized leaves known as bracts are found at the basis of flowers or flower stalk. In Christmas flower (poinsettia), the flowers have no petals, but the brightly colored floral bracts that surround the small flowers make up for the absence of petals. The red-colored bracts The terminal leaflets Reproductive leaves (modified leaves) of this garden pea surrounding the true plant leaf are flowers. modified as tendrils. ❑ Storage leaves: Some desert plants have succulent leaves (leaves retain water). The modification that permit water storage, involve large, thin walled parenchyma cells without chloroplasts to the interior of chlorenchyma tissue just beneath the epidermis. ❑ Spines, Thornes, and prickles ▪ Spines are modified leaves in many desert plants. In several woody plants, the stipules at the bases of the leaves are modified as short, paired spines. ▪ Thorns: modified stems, they are spinelike objects arising in the axils of leaves. ▪ The prickles of roses, however, are neither leaves nor stems but are outgrowths from the epidermis or cortex just beneath it Storage leaves The spines Prickles are Thorns modified outgrowths from (modified leaves the epidermis or stems). the cortex.