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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Flower is a Modified shoot-digieduco

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Flower is a Modified shoot- That the flower is a modified shoot or a branch in the very beginning can be revealed by the following facts :

Flower is a Modified shoot-digieduco
1. AXIS NATURE OF THALAMUS-Flower consists of an axis known as thalamus or torus.

(a) This axis bears modified floral leaves viz, sepals, petals, stamens and carpels. Generally the axis in a flower consists of short or suppressed internodes and nodes. But it is not always the case ; the axis is long in some flowers e.g. in Gynandropsis gynandra(Capparidaceae), Passiflora suberosa (Passifloraceae) etc. the internodes of the thalamus between corolla and androecium (called androphore) and between androecium and gynoecium (called gynophore) become enlarged with same whorled arrangement of stamens and carpels respectively ; so no doubt that the thalamus is axial in nature. In Capparis
sepiaria (Capparidaceae) the formation of gynophore only is also noted.
Flower is a Modified shoot-digieduco

(b) Normally, the growth of the thalamus is checked by the carpels but in some cases like species of Pyrus (pear), Rosa (rose), etc., the thalamus had been found to undergo further upward growth having green stem with prickles and small foliage leaves beyond the gynoecium-this phenomenon is known as monstrous development or proliferation.
Flower is a Modified shoot-digieduco

(c) In Polyalthia longifolia (Anonaceae), Michelia champaca (Magnoliaceae) eHoral the carpel bearing region of the thalamus elongates like the stem giving rise to an aggregate fruit.

2. LEAF NATURE OF FLORAL MEMBERS-That the floral leaves are modified leaves is proved in most cases by their leafy nature ; in many cases the sepals may assume typical leaf-like structure with veins and petal-like pigments e.g. Mussaenda frondosa (Rubiaceae).

The floral leaves also exhibit the same types of ptyxis and aestivation Le. prefoliation as those of foliage leaves.

In Michelia champaca, Magnolia sp. etc. of Magnoliaceae the floral leaves are both vertically and spirally arranged on long thalamus but in majority of flowers the floral phyllotaxy i.e. arrangement of floral leaves is whorled- this resembles the phyllotaxy of foliage leaves on the stem and branches.

The leaf nature is also proved by the gradual transition of sepals to petals and petals to stamens as seen in flowers of Nymphaea sp. (Nymphaeaceae). The transition from leaves to sepals and from sepals to petals showing  similar leafy structure is also noted in Paeonia sp. (Ranunculaceae).

In double flowers raised by horticultural skill e.g Rosa sp. (Rosaceae), Hibiscus rosa-sinensis (Malvaceae), the stamens are further remodifled into petals-so stamens are highly modified leaves.
Flower is a Modified shoot-digieduco

In Canna sp. (Cannaceae), stamens are modified into petaloid staminodes. In Zinnia Sp. carpels are modified into sepaloid or petaloid structures.

That the carpel is also leafy in nature can be proved on examination of single carpel of pisum sativum (Papilionaceac) in which it is folded along its midrib to form a chamber (the ovary) containin g seeds. The elongated part of the carpellary leaf forms the style ; its extreme apex, the receptive spot for the pollen forms the stigma.

3. HOMOLOGY OF FLORAL BUDs-Floral buds like vegetative buds may be metamorphosed bulbils e.g. viviparous bulbils of Agave sp. (Agavaceae). Development and position of vegetative buds are also like those of floral buds i.e. axillary and terminal.







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