Plant anatomy and outline structure of plant body-digieduco

PLANT ANATOMY is that branch of botany which deals with the study of internal structure and organization of different plant groups. The word 'anatomy' has derived its origin from ‘ana ’ meaning asunder, i.e. to pieces and 'temnein‘ meaning to cut. As anatomy deals with me internal structures of different organs so it is also called internal morphology in general. The term ‘histology‘ is often misused for anatomy, as histology deals only with the study of minute internal structures of tissues and their development (Sass, 1958), but not the gross internal structures like steles, nodal anatomy, vascular skeleton etc. The first important investigators in plant anatomy are English scientists Robert Hooke (1635-1703), Nehemiah Grew (1641-1712) and an Italian physician Marcello Malpighi (1628-1699).They Published their contributions to the knowledge of internal plant, structure in the later part of the 17th century. Since then work on anatomy regarding the existence of the living cell contents, cell theory, ontogenetical studies of different plant organs etc. have been carried out by various plant anatomists from time to time .

Outline structure of the plant body : The outline structure of plant body develops from a zygote Which results from the fertilization of a female gamete by a male gamete. At the beginning, the zygote divides into two cells which themselves undergo further divisions forming the embryo. In vascular plants, i.e. pteridophytes and spermatophytes, the mature plant develops from the embryo as a result of differentiation through a long period.

Although there is great diversity in size, form and structure among vascular plants there is a fundamental and uniform structural plan. The plant body fundamentally consists of a cylindrical axis bearing lateral appendages. This simple and uniform structural plan of the axis remains concealed often due to free branching of the axis and the complexity and variety of the appendages.

Main parts of the Plant body-Though the axis is a continuous structure. Still, morphologically and physiologically, it consists of two distinct regions, e.g. upper aerial portion known as stem and the subterranean portion known as root.

The root-stem axis bears appendages of three types, e.g. (a) leaves, (b) emergences and (c) hairs. The appendages in which vascular strands pass from the axis are known as leaves ; leaves are characteristic of the stem and are absent in roots. From the anatomical point of view the leaves may be looked upon as lateral expansions of the stem. Leaves are arranged on the stem in a definite manner and bear an intimate structural relation to the skeleton of the axis. The second type of appendages are the emergences which deveIop only from the outermost layers of the stem i.e. from the cortex and the epidermis ; prickles of rose are the best examples of the emergences. The third type of appendages are the hairs developing from the outermost layer of the cell only. Emergences and hairs occur in both axis and leaves, but usually without definite arrangement.

The axis-The axis consists of a central core with a surrounding layer. This central core contains the greater part of all the supporting and conducting cells. i.e.. mechanical and vascular tissues and therefore the core serves mainly the important functions of conduction and support. The central core, i.e. the central unit mass is known as the central cylinder or stele owing to its central position and shape in the axis. The surrounding ensheathing layer is called cortex. the outermost layer of which forms the epidermis. The cortex serves the function of protection, storage and support.

The stele is primarily composed of two types of vascular tissues, viz. the xylem and the phloem xylem conducts water and other substances absorbed from the soil, while phloem carries the food and mineral nutrients to different plant organs. In most cases xylem and phloem always occur together, either side by side radially (in roots) or on the same radius (in stems and leaves) forming bundle-iike structure called vascular bundle. The vascular bundles of dicotyledons and gymnosperms contain a lateral meristem between xylem and phloem that lateral meristem is called cambium. In addition to vascular tissues, the stele may also contain non-vascular tissues. According to the arrangement of vascular and non-vascular tissues the stele may be solid, rod-like, a hollow cylinder etc. Sometimes the vascular tissues enclose a soft and loose tissue (parenchyma) in the central region of the stele, that region is called pith or medulla continuation of parenchymatous tissue in radiating manner from the pith in between vascular bundles may be present in the stele -these are called medullary rays orpith rays. Outside the external conducting cells, i.e vascular bundles, one or more layers of non-conducting cells occur forming the outermost layer of the stele-this outermost layer is known as pericyle. The stele is ensheathed externally by pericycle.

Primary and Secondary body-The axis with all the above mentioned structural features including complete appendages constitutes the fundamental parts of the plant body. This body is built up by the growth and activities of the primary tissues derived from the apical meristem present at the growing points, i.e. at the apices of the axis. Thus the first-formed plant body composed of primary tissues is called primary body. As the primary body is built up first by the activities of primary tissues hence this process of growth is called primary growth. Primary growth builds up the new or young parts of the plant body; as a result of primary growth, the axis grows in length, appendages of the axis develop and its branch system is laid down. After the growth thus formed has attained maturity, further growth i.e. increase in girth or thickness takes place only by a process called secondary growth. Secondary growth takes place by the formation of secondary tissues in the plant body. In the stele, secondary vascular tissues are formed by the activities ofprimary lateral meristem called fascicular cambium (which is present between the primary xylem and primary phloem of the vascular bundles of dicotyledonous and gymnospermic stems) and the secondary lateral meristem called interfascicular cambium. These two cambia join with each other to form a complete cambium ring. That cambium ring adds new tissues like secondary xylem and secondary phloem, entirely within the stele. After formation of these secondary tissues in the stele, the secondary growth in the cortex starts by the formation of a new lateral meristem (secondary) called phellogen or cork cambium. Phellogen arises in the peripheral region of the cortex, this meristem gives rise by division to phellem or cork cells on the outer side and to phelloderm (parenchyma cells) i.e. secondary cortex on the inner side. In this way, the axis grows in thickness or girth due to the formation of secondary tissues by the above mentioned processes. As the growth in thickness takes place due to addition of secondary tissues, so this type of growth is called secondary growth and the plant body composed of secondary tissues is referred to as secondary body.

Composition of the plant body-The vascular plant has a highly evolved body having structural and functional specialisation. This structural and functional specialisation of the plant body is expressed in the differentiation, externally into organs and internally into various categories of cells, tissues and tissue systems. Three vegetative organs, viz. the root, the stem and the leaf are generally recognised in spermatophytes. The flower is interpreted as an assemblage of organs, some of which are reproductive (stamens and carpels), others are sterile (sepals and petals). Cells are the ultimate units of plant structure. A tissue is a group of more or less alike cells having same function and same methods of development. Tissues having restricted functions are again organised into large entities to form plant organs-such organised tissues are referred to as tissue systems. Hence regarding internal structure, one should give stress on the characteristic features of cells and tissues and then establish types on the basis of distinctions of organs.