Lecture 1: External Form of 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.