An Outline of the Plant-Body (With Diagram)!
The plant itself originates from the morphologically unicellular zygote, which in course of time develops into the embryo and then to the mature plant. The higher plant is the result of evolutionary specialisation through a long period.
In spite of great diversities existing in the vascular plants as regards size, structure and form, there is a fundamental uniformity. The structural plan of a higher plant (Fig. 480 A) is such that each plant has an axis, the lower part of which is the subterranean root and the continuous upper aerial part is the stem.
The root-stem axis bears appendages, which are of three types. The most important ones are those in which vascular an outline of the plant-body strands passes from the axis. Leaves are such appendages. They are characteristics of the stem and are absent in the roots. Leaves have a definite order of development and are in close association with the stem. Thus they are regarded as lateral expansions of the stem.
The root, stem and the leaves are the three main organs of a plant. These three organs are intimately related both from developmental and structural points of view. The flowers are reproductive organs which come out after the plants have attained maturity.
It is an accepted view that the flower is homologous to a shoot and floral parts with the leaves. The second type of appendages, called emergences, develops from the external portions of the stem, the epidermis and cortex usually taking part in their formation.
The hairs arising from the outermost layers of the stems and the leaves constitute the third type of appendage. These are superficial outgrowths. Emergences and hairs, unlike the leaves, have no definite order of development.
The Axis:
The root and stem from a continuous axis of the higher plant. Three zones or regions may be usually located in the axis (Fig. 480 C & E). The epidermis of the skin forms the outermost zone. It is usually uniseriate due to anticlinal (at right angles to the axis) division of the mother cells, and is protective in-function. The second zone internal to epidermis is the cortex.
It is many layered and is composed of different types of tissues performing functions like support, storage, etc. The last layer of cortex is the endodermis which is also normally uniseriate. Next comes the central core, known as stele or central cylinder, which practically remains enveloped by the cortex. Stele includes the vascular bundles, consisting of complex tissues, xylem and phloem, and other tissues.
Functions of vascular bundles are twofold—firstly, conduction, and, secondly, mechanical support. Xylem is meant for conduction of water with dissolved mineral matters absorbed from the soil and also for mechanical purpose, while the other tissue phloem is there for conduction of elaborated food matters in solution.
A lateral meristem, called cambium, may be present between xylem and phloem, as in the stems of dicotyledons and gymnosperms. Xylem and phloem may remain radially, as in the roots, or they may occur on the same radius, as in other organs.
According to the arrangement of the tissues the stele may be like a solid rod, a hollow cylinder, etc. In some cases they enclose a soft parenchymatous portion in the central region, which is known as pith or medulla.
Continuations of parenchymatous pith passing in between the vascular bundles may look like rays radiating from the pith. They are called pith rays or medullary rays. Outside the vascular bundle occur one or more layers of non-conducting cells forming what is known as pericycle. It lies internal to endodermis and forms the outermost sheath of the stele.
Primary Body and Secondary Body:
The axis with the appendages constitutes the fundamental part of the plant. All the tissues forming this body are the derivatives of the apical meristem present at the tips or apices of the axis (Fig. 480 B & D).
These tissues are called primary tissues, and the body composed of primary tissues is the primary body. During the formation of this body, what is called primary growth, the axis grows in length, appendages develop and the branch systems are laid down.
Lateral meristems like cambium occur between primary Xylem and primary phloem in the vascular bundles of dicotyledonous and gymnospermic stems.
Those cells divide and produce new tissues—secondary xylem and secondary phloem between primary xylem and primary phloem. After formation of these secondary tissues a new meristem, called phellogen or cork cambium, arises in the peripheral portion of the organ.
This meristem which is also lateral, cuts of phellem or cork cells with suberised walls on the outer side and parenchyma cells called phelloderm or secondary cortex on the inner side. As a result, the axis grows in girth or thickness.
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 organs like stem, root and leaves. These are distinct morphological structures, each having specific function to perform. Every organ, in its turn, 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 group of cells formed in response to a basic division of labour. So the ultimate units of the plants are the cells. Thus it is customary to begin studies on anatomy with the cell.
