In this essay we will discuss about the general structure of the plant body, explained with the help of a diagram.
Growth of the plant body starts generally from the morphologically unicellular zygote which gradually develops into the embryo and then to the whole plant. In spite of great diversities existing in the vascular plants as regards their size, structure and form, there is a fundamental uniformity. The higher plant body generally has an axis, the lower part of which is the underground root and the continuous upper aerial part is the stem.
The root-stem axis bears appendages, which are of three types – the leaves, emergences and hairs or trichomes. The leaves are characteristic of the stems and absent in roots. They have the connections with the stem vasculature. Thus the root, stem and leaves are the three fundamental organs of the plant body.
Starting from the periphery the plant axis has three zones or regions – the epidermis, cortex and stele. The epidermis is the outermost layer forming the skin. It is usually single-layered due to anticlinal (at right angles to the axis) division of the mother cells and gives protection to the plant body.
The next internal zone to epidermis is the cortex. It is multi-layered and is composed of different types of tissues performing functions like support, storage, etc. The innermost layer of the cortex is the endodermis. The central core zone is the stele or central cylinder, which remains encircled by the cortex.
Stele includes the vascular bundles, consisting of complex tissues like xylem and phloem, and other simple tissues. The vascular bundles perform the function of translocation of water and solutes as well as give mechanical support to the plant.
Xylem translocates water along with dissolved mineral matters absorbed from the soil by roots and also forms the mechanical tissue system inside the plant. Phloem, on the other hand, is responsible for the translocation of prepared food matters from the leaves to the different parts of the body.
A lateral meristem, called cambium, may be present in between the xylem and phloem, as in the stems of dicotyledons and gymnosperms. Xylem and phloem remain arranged radially in the roots and occur on the same radius in other organs. The stele may form a solid rod, a hollow cylinder, etc.
Very often they enclose a central, soft parenchymatous pith or medulla, which is continuous in-between the vascular bundles looking like rays radiating from the pith — called pith rays or medullary rays. In between the vascular bundles and endodermis there occurs one or more layers of non-conducting cells forming what is known as pericycle.
All the tissues forming the plant body are derived from the apical meristem present at the tips or apices of the axis. These tissues are called primary tissues, and the body composed of primary tissues is the primary body. During primary growth, the axis grows in length, leaves, axillary buds and other structures develop originating the branch systems.
Cambium in-between primary xylem and phloem in the vascular bundles of dicotyledons and gymnosperms divide and produce new tissues – secondary xylem and secondary phloem – in the later stages of growth.
After formation of these secondary tissues a new meristem, called phellogen or cork cambium, arises in the peripheral portion of the stem This lateral meristem cuts phellem or cork cells with suberised wall on the outer side and parenchymatous phelloderm or secondary cortex on the inner side.
As a result, the axis grows in girth or thickness. All these tissues, formed later by the lateral meristems, are known as secondary tissues; and the growth in thickness due to addition of secondary tissues is referred to as secondary growth.
Thus plant body is composed of morphologically distinct organs like stem, root and leaves, each performing specific functions. Every organ, again, is a collection of tissue-systems which carry on restricted functions.
A tissue-system is made up of a number of contiguous tissues. A tissue is, in fact a collection of cells formed in response to a basic division of labour. So the ultimate units of the plant body are the cells. Thus it is customary to begin studies on anatomy with the cell.