Cell membrane function and difference-digieduco

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  Functions of cell membrane : (a) The proteins and enzymes present in the cell membrane helps in the transport of certain substances like sugar, sodium and other ions etc. across the cell membrane. (b) Cell membrane protects the internal structures of the cell and different organelles of the cytoplasm. (c) It maintains the shape of the cell. (d) It acts as a selective permeable membrane because it allows to pass certain substances while others are not, hence it helps in the transport of selective materials from and to the cells. (e) The membranes on the cytoplasmic organelles and nucleus most probably formed from the plasma membrane. (f) Other important functions of cell membrane of the cells are endocytosis and exocytosis . Endocytosis is the process by which materials are transported into the cells by the formation of vesicles. It includes two processes-phagocytosis (cell eating) and pinocytosis (cell drinking). The reverse process of endocytosis is exocytosis. During this p
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Leaf: insertion of leaves, phyllotaxy,and Introduction of spiral phyllotaxy-digieduco

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The leaf

Insertion of Leaves : The point at which a leaf is attached to the stem or branch is called its insertion. Leaves are variously inserted on the stem and branches and accordingly different terms are given. Leaves are called radical when they arise at or below the surface of the ground and thus spring apparently from the root, but really from a shortened stem ; Examples-Taraxacum officinale (Compositae), Primula vulgaris (Primulaceae), species of Xyris (Xyridaceae) etc. The leaves which arise from the main stem with distinct internodes between them are called cauline, and those from the branches ramal.

Phyllotaxy : Phyllotaxy is the mode of arrangement of leaves on the stem and the branch. The main principle of this arrangement of leaves on the stem and the branch is to avoid shading one another so that each and every leaf may get maximum amount of sunlight to perform their normal function, Eames and MacDaniels (1947) have also stated that “leaf arrangement on the stem, or phyllotaxy, and variations in size and shape are doubtless in part correlated with the exposure of the photosynthetic surface to light.” This arrangement of leaves in a definite manner may also bear an intimate structural relation to the vascular, skeleton system of the axis.


Leaf: insertion of leaves, phyllotaxy,and Introduction of spiral phyllotaxy-digieduco
Different types of phyllotaxy

Phyllotaxy is of three principal types viz : ( i ) spiral or alternate or acyclic, (ii) opposite and (iii) whorled 0r verticillate. Opposite and whorled or verticillate types are commonly called cyclic type of phyllotaxy.

1. SPIRAL OR ALTERNATE PHYLLOTAXY (i.e. acyclic type )In this type, leaves are arranged at each node in a spiral manner all round the stem. If an imaginary Spiral line is drawn winding round the stem, it will pass through consecutive nodes. Hence it is termed as spiral phyllotaxy. This imaginary spiral line is known as genetic spiral. When imaginary vertical lines are drawn from upwards and downwards, it is found that all the leaves of the plant lie in a fixed number on these vertical lines ; these vertical straight lines passing through leaves are known as orthostichies (singular, orthostichy). The distance formed upon the spiral of the stem between two successive leaves is called divergence. The angle sebtended by the divergence at the centre is known as angular divergence. The angular divergence can be found out by the following formula:

The angular divergence=number of circles or circles/no of leaves or orthostichies of a circle i.e.360°.


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