عدد المساهمات: 5918
تاريخ التسجيل: 01/10/2009
|موضوع: اقرأ كتاب تشريح النبات باللغة الانجليزية من النت مباشرة دون تحميل الإثنين يناير 11, 2010 3:43 pm|| |
The science of the structure of the organized plant body learned by dissection is called Plant Anatomy (anatomy-dissection). In general, Plant Anatomy refers to study of internal morphology, pertaining to different tissues. The subject of this chapter is structure of Angiosperms, with emphasis on primary tissues.
Plant body in Angiosperms is differentiated into root stem, leaf and flower. All these parts are made up of different types of tissues containing different cell types. A tissue is a mass of similar or dissimilar cells performing a common function.
The body of a vascular plant is composed of dermal tissue, Ground tissue and Vascular tissue.
Dermal Tissue (Skin)
Dermal Tissue is protective in function. Basing on its origin, it is classified into two types – Epidermis and Periderm.
Epidermis:This is the primary surface tissue of the entire plant. Epidermal cells are compactly and continuously arranged; the continuity is lost by the presence of Stomatal pores or breaks in the tissue. Covering the aerial epidermis, cutin (fatty substance) is present as an impregnation on cell wall. The cuticle can be separated from epidermis. The epidermis may produce unicellular or multicellular hairy outgrowths and other appendages. Epidermis provides mechanical protection, allows gaseous exchange through stomata, restricts transpiration with cuticle, and is also involved in storage, photosynthesis, secretion, absorption and perception to stimuli. Stomata are Pores, each guarded by two guard cells, which control the size of the pore. Cells surrounding guard cells, but differing from other epidermal cells, are called subsidiary cells. Guard cells are kidney shaped; their cell walls are thick on the inner surfaces. Guard cells contain many chloroplasts.
Periderm:This is formed during secondary growth replacing primary epidermis.
This is inner to dermal tissue and is composed of simple tissues like parenchyma.
Vascular tissue consists of conducting elements – xylem and phloem. Vascular tissue may be scattered in ground tissue or regularly arranged forming a ring. In the latter arrangement, ground tissue is differentiated into cortex (outer to vascular tissue) and pith (inner to vascular tissue). The ground tissue of leaves is called mesophyll, bound by upper and lower epidermis.
In broad sense, tissues are classified as – meristematic and permanent tissues.
Meristem (Meristos – divisible)
Initially all embryonic cells of an embryo have the capacity to divide and multiply but as the embryo develops into a plant body, this capacity for division is restricted to certain parts of the plant body called meristems which are active throughout the life of the plant body (unlike that of an animal body). When meristematic cells divide, a group of the daughter cells remain meristematic; the other daughter cells called derivatives differentiate into various tissue elements. Before the occurrence of any cell division, usually cells become enlarged accompanied with addition of protoplasmic and cell wall material.
Meristematic cells are isodiametric, compactly arranged with dense cytoplasm, large nucleus, and small vacuoles or without vacuoles. Cell walls are thin.
Meristems which occur at the apices of stem, root and other branches are called apical meristems, which bring about primary growth of the plants, hence also called as primary meristems. In many plants in addition to apical meristems, lateral meristems like vascular cambium, cork cambium, intercalary meristems are found. Lateral meristems are arranged parallel to the sides of organs in which they occur.
Intercalary Meristem: This is also a primary meristem, found inserted between permanent tissues, in the bases of internodes and leaf sheaths of grasses. Sometimes, as in gynophore of groundnut, xylem may be present in intercalary meristem. Wherever stem is jointed, elongation of internodes is due to intercalary meristem. Example: Bamboos. Even prolonged growth of leaves, flowers and fruits may be regarded as an intercalary growth.
Vascular cambium and cork cambium: These are referred to as secondary meristems because they produce secondary tissues, and increase the thickness of the plant body. This process is called secondary growth, seen in dicotyledons and gymnosperms.
Permanent or Mature Tissues
Cells derived from meristems gradually change in their structure, metabolism and chemistry and acquire specialized characters by their various modes of differentiation. Not all the cells totally differ from the meristems. Some cells retain the power of division and others cannot divide. In a strict sense only cells which have lost the power for division must be regarded as permanent tissues, but in a broad sense, cells derived from meristem that have acquired a special function like photosynthesis, secretion, storage are treated as part of matured tissue. There are different types of mature tissues. Example: Parenchyma, Collenchyma, Sclerenchyma, Xylem and Phloem.
Types of Mature Tissues
Parenchyma (Para-beside, enchyma-In poured)
Parenchyma is the fundamental tissue of the plant body. It is found in every part of the plant body like pith and cortex of stem and root, mesophyll of leaves, flesh of fruits, floral parts and even in xylem and Phloem. Cells have thin primary walls and polyhedral shapes. The average number of faces of a polyhedron is 14 and is called as "tetra decahedron". Cells are compactly arranged or more commonly spaciously arranged with intercellular spaces as in cortex and pith. Cells possess dense cytoplasm and are active metabolites.
Generally cells of parenchyma are involved in storage of starch, sucrose, protein, water, phenol derivatives, many mineral substances, etc. Other metabolisms like respiration, protein synthesis etc., are active. Parenchymatous cells may also perform specialized functions and are structurally modified. The following are the different types of parenchyma.