Cell membrane function and difference-digieduco

on
  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
0

Leaf: Definition, parts of typical leaf and kinds of leaves-digieduco

on
The leaf

Definition : Leaves are flattened lateral dissimilar appendages born on the stem  and branches. They are usually green in colour and are developed in acropetal order from the nodes (the place from where a leaf originates and it comprises the point of attachment of the leaf round the stem or branch). The leaves bear buds at their axils and are exogenous -in origin.


Parts of a Typical Leaf  : A typical foliage leaf has three parts e.g.
(1) Leafbase or hypopodium, (2) the petiole or mesopodium or the stalk of the leaf and (3) Leaf lamina or leaf blade or epipodium.
Leaf: Definition, parts of typical leaf and kinds of leaves-digieduco
Parts of typical leaf

(i) Leafbase or Hypopodium-It comprises the point of attachment of the leaf to the stem. As the tissue of hypopodium and that of the stem are continuous it is difficult to delimit accurately the hypopodium from the stem ; very often the point of union of the two is characterised by a swelling of the hypopodium forming a type of cushion known as pulvinus. The cushion forms a kind of sheath, almost surrounding the stem, as in Pelargonium sp. (Geraniaceae). In most monocots, the leaf-base Clasps and sheaths the stem forming sheathing leaf-base.

(ii) Petiole or Mesopodium or Leaf stalk-It is usually a long or short cylindrical non-membranous part of the leaf in direct continuation of the leaf base. The petiole is present in most leaves and then the leaves are known as petiolate leaves ; in some leaves petioles are apparently absent, such petioleless leaves are known as sessile leaves. Examples of sessile leaves are found in Argemone mexicana (Papaveraceae), Canscora diffusa (Gentianaceae), etc. Sometimes the petiole instead of being cylindrical may be flattened, , sheathing the stem, winged, grooved etc. Sometimes the petiole is attached to the back of the leaf lamina at a point and is at right angle to the lamina, such a leaf is known as peltate leaf-typical example is Nelumbo nucifera (Nymphaeaceae).

(iii) Lamina or Epipodium or Leafblade-It is usually the thin, membranous, green, expanded portion of the leaf and comprises the greater part of the leaf. The part of the axis of leaf (phyllopodium) which divides leaf blade into two symmetrical parts very often, is known as the midrib.
Veins are the branches and branchlets of the midrib of the leaf ; lateral slender branches of the first order of the midrib are known as primary veins. The primary veins again repeatedly branches and in many cases form a network. giving rise to veinlets which carry food and water conducting elements.
Leaf: Definition, parts of typical leaf and kinds of leaves-digieduco
Cotyledonary leaf of Ricinis communist and stem of Asparagus showing cataphylls

 Kinds of Leaves : (a) Foliage leaves-These are the common green leaves which are concerned with the photosynthesis, respiration and transpiration. These leaves are lateral appendages of the stem or the branch, and develop at the node.

(b) Seed leaves or Cotyledonary leaves i.e. the leaves of embryo forming a first pair of leaves during germination, these leaves are attached to the axis of the embryo of the seed.

(6) Scale leaves or Cataphylls are thin scaly or membranous structures, they occur on underground stems and branches e. g. rhizome, corm etc. ;such membranous scale leaves may be present occasionally on stolon e.g. the stolon of strawberry (Fragaria vesca, Rosaceae). Scale leaves are sometimes found growing in clusters on main stem or on branches of ordinary plants, e.g. many plants of the family Euphorbiaceae, Liliaceae  etc. Morphologically scale leaves or cataphylls are the reduced form of foliage leaves.

(d) Bract leaves or Hypsophylls are the special leaves, either modified in structure and colour or may be ordinary foliage leaves reduced in form, developing at the base of individual flowers or florets or the inflorescence.

(e) Floral leaves and Sporaphylls-These are the leaves of a flower, and are modified foliage leaves. Sepals, petals, perianth etc. are the floral leaves. Sporophylls are spore bearing leaves concerned with reproduction. Stamens and carpels are the sporophylls of angiosperms.

(f) Prophylls-The first few formed leaves on a stem or a branch differing from Other leaves ane termed prophylls. Generally one prophyll is found in monocots and two prophylls in dicots. Sometimes prophylls may be modified into spines (e.g. Aegle marmelos) and tendrils (e.g. Cucurbita sp.). Prophylls do not subtend floral buds.



Comments

Post a Comment