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 th...
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Leaf: spiral phyllotaxy and its kinds-digieduco

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Spiral phyllotaxy

Spiral phyllotaxy is of several kinds, such as (a) distichous or 1/2 phyllotaxy,

(b) tristichous or 1/3 phyllotaxy, (c) pentastichous or 2/5 phyllotaxy, (d) octastichous or 3/8 phyllotaxy etc.

Leaf: spiral phyllotaxy and its kinds-digieduco

(a) Distichous or T wo-ranked or phyllotaxy-It is a simple type found in all grasses (Gramineae e.g. Cynodon dactylon, Oryza sativa, Triticum aestivum, etc.) and Vanda roxburghii (Orchidaceae). In this type, the third leaf base falls vertically above the first, . the fourth above the second, the fifth above the third and so on. So, there are only two orthostichies on which all the leaves are placed. Upto the base of the third leaf there is one complete turn round the stem on the genetic spiral which includes two leaves ; hence the leaves are equidistant .Therefore any two consecutive leaves are placed 1/2 the distance of a circle, this is constant ; so the angle subtended by two consecutive.
Leaf: spiral phyllotaxy and its kinds-digieduco
transverse projection

(b) Tristichous or three-ranked or 1/3 rd phyllotaxy-This type is found in all the members of the family Cyperaceae (Cyperus rotundus,etc). The first, fourth, seventh leaf bases are on one orthostichy ; the second, fifth, eighth leaf bases on the second orthostichy;third, sixth and nineth on the third orthostichy. The three consecutive leaves at different nodes are equidistant i.e. upto the base of the fourth leaf there is one complete turn or circle which includes three leaves. Three orthostichies are found in this case. As the divergence between any two consecutive leaves being 1/3 rd of a circle, the angular
divergence is C-H 1/3 of 360° = 120°.

(c) Pentastichous or Five-ranked or -2/5th phyllotaxy-This type of phyllotaxy is found
in Hibiscus rosa-sinensis (Malvaceae), Ficus benghalensis (Moraceae), etc. In this case the sixth leaf base stands vertically above the first, the seventh leaf base above the second, the eighth leaf base above the third and so on ; upto the base of the sixth leaf there are two complete turns including the leaves: Hence any two consecutive leaves are placed at 2/5th distance of a circle. Here five orthostichies are found, therefore the angular divergence is2/5-0f 360°= 144°.

( d ) Octastichous or eight-ranked or 3/8th phyllotaxy-This type of phyllotaxy is noticed
in plants like Hibiscus mutabilis (Malvaceae), Carica papaya (Caricaceae), Thevetia peruviana (Apocynaceae), etc. Here the nineth leaf base falls vertically above the first, the . . tenth above the second, the eleventh above the third and so on ; there are eight orthostichies on which all the leaves are inserted. The consecutive leaves are placed at 3/8 th distance of  a circle, as upto the base of the nineth leaf there are three complete turns or circles which include eight leaves, therefore the angular divergence is 3/8 of 360° = 135°.
Leaf: spiral phyllotaxy and its kinds-digieduco
Parastichous type of phyllotaxy

Parastichous is a type of phyllotaxy when orthostichies are not possible to count owing to a big fraction of the divergence. In this type, many leaves are crowdedly situated on nodes and the internodos are very short, e.g.phoenix sp. (Palmae)

2. OPPOSITE PHYLLOTAXY-In this type of phyllotaxy two leaves are inserted at each node opposite to each other ; it is of two kinds-

(i) Opposite decussate-When one pair of leaves of one node stands at a right angle to the next upper and lower pair of leaves, it is called opposite decussate e.g. Ixora arborea (Rubiaeeae), Calotropis procera (Asclepiadaceae), Ocimum sanctum (Labiatae), etc.

(ii) Opposite superposed-When the opposite consecutive pairs of leaves are arranged in one plain i.e. serially arranged on top of one another not forming a cross, it is called opposite superposed, e.g. Psidium guajava (Myrtaceae), Quisqualis indica, Hiptage benghalensis (Malpighiaceae) etc.

3. WHORLED PHYLLOTAXY-In this type of arrangement three or more leaves are arranged at each node in a circle or whorl round the stern. Leaves are arranged at each node verticillately. The leaves are so nicely arranged that the respective leaves of the whorls of upper and next lower fall between the inter spaces, so that no over crowding of leaves takes place, e.g. Allamanda sp., Alstonia scholaris, Nerium indicum (Apocynaceae), etc.




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