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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Stem: Definition, characteristics, forms and surface of stems- digieduco

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The stem

Definition: A stem is an ascending organ usually developed from the plumule and grows vertically up; sometimes it is sub- aerial or underground.

Stem: Definition, characteristics, forms and surface of stems- digieduco
Portion of a stem apex showing different parts

The plumule ,developed after the germination of the seed, elongates upward forming the shoot i.e. leaf bearing axis or stem. The stem bears branches (exception: : Palms, Cycas, etc.) and leaves which are together called shoot. Modified reproductive shoots are found on the stem, known as flowen.A stem growing above the ground is generally green in young condition but due to the occurrence of secondary growth and formation of bark, the green colour of the stem changes at the adult stage. Stems growing under the soil are always non-green in colour.

Characteristics of the Stem : Stem is the ascending part of the plant axis, developing from the plumule. Stem bears leaves, branches and flowers. It is provided with nodes and internodes. The region on the stem or branches where one; more leaves arise is known as node, and the space between two successive nodes is cam the internode. The angle formed between a leaf and the internode is termed axil; one or more lateral i.e. axillary buds are present at the axis of leaves. The stem surface often bears multicellular hairy outgrowths. The terminai portion of the stem is occupied by a bud called apical or terminal bud. The colour of the stem is usually green.

Read also-Roots: Definition,characteristics, types and regions of roots-digieduco
Roots: modified roots and functions of root-digieduco

Forms of stems: The form of the stem varies in different plants though generally it is cylindrical in appearance. Following are the different forms of stem:

(i) Square i.e. stem is four-angled forming a square structure, e.g. Leonurus sibiricus (stiatae), Pen'stmphe bicalyculata (Acanthaoeae), Cissus quadrangularis (Vitaceae) etc.

(ii) Triangular i.e. stem is three-angled e.g. Cyperus rotundus (Cyperaceae).

(iii) FIat-In this type the stem is flat and leaf-iike, e.g. Opuntia diuenii (Cactaceae).

(iv) Jointed or Articulated--In this type, joint or articulation appean in the node which bulges extemally forming a hardened ring. e.g. sugar cane (Saccharum officinarum-Gramineae) and other grasses.

(v) Ribbed i.e. stem with ridges and furrows, found in species of Cucurbita (Cucurbitaceae), Casuarina (Casuarinaceae) etc.

(vi) Ribbon~shaped---In this type the stem is flat filamentous or ribbonvshaped, e.g. Podostemon sp. (Podostemaceae).

Usually a stem is solid but due to the disorganisation of the pith i.e. central region it
may be hollow or fistular as seen in many grasses, Cucurbita sp. Etc.

 Surface of Stems : The surface of the stem may be of different nature, viz:

(a) GIabrous-In this type, the surface of the stem is smooth i.e. without any emergences e.g. Boerhaavia repens (Nyctaginaceae), Hibiscus rosa-sinensis (Malvaceae) etc.

(b) Glaucous-In this type, the stem is smooth and more or less shining c.g. Solanum glaucum (Solanaceac), Zea mays (Gramineae) etc. .

(c) Pubescent or Tomentose-Here the stem is provided with various types of hairs e. g. woolly, rough etc. c. g. Helianthus annuus (Compositae), Clerodendron infortunatum (Verbenaceae), Nerium odorum (Apocynaceae).

(d)Prickly or Spiny stem-Here the stem is provided with spines and prickles e. g. Opuntia dillenii (Cactaceae), Rosa centifolia (Rosaceae), Bougainviilea spectabilis (Nyctaginaceae), Solanumferox (Solanaceae) etc.

 According to the nature, duration and mode of branching of stems, plants are divided into Herbs, Shrubs, Undershrubs and trees:

A. HERBS OR HERBACEOUS PLANTS :

Plants which are erect, smaller in size and have soft stems are called herbs. Generaliy they are unbranched, branching if present are few in number. According to the duration of life, herbs are further classified into ephemerals, annuals. biennials and perennials.

(i) Ephemerals .'--These plants live only for a few days ; within these few days they have to complete their life cycle, e.g. Balanites aegyptica (Zygophyllaceae).

(ii) Annuals :-They live only through one season and complete their life cycle within that period, e.g. Oryza sativa (Gramineae), Brassica nigra (Cruciferae), Triticum aestivum ‘ (Gramineae) etc.

(iii) Biennials:-They live through two successive seasons. During the first season they germinate from the seeds and produce vegetative parts and in the second season they die after producing flower, fruit and seed e.g. Daucus carota var. sativa (Umbelliferae),
Raphanus sativus (Cruciferae) etc.

(iv) Perennials .---'They live and complete their life cycle in more than two seasons,e.g. Scoparia dulcis (Scrophulariaceae), Zingiber officinale (Zingiberaceae). Borreria hispida (Rubiaccae) etc.

B. SHRUBS-These are medium sized erect small plants with woody stem. The hard stem is profusely branched and branching is so profuse that it is very difficult to distinguish the main stem. Common examples are : Hibiscus rosa-sinensis (Malvaceac), Duranta
repen: (Verbenaceae), Jasminum sambac (Oleaceae) etc. In shrubs trunk is absent.

 C. Under shrubs-plants whose characters are found in between shrubs and herbs called under shrubs e.g. solanum melongena, capsicum frutescens etc. Of the family solanceae.
Stem: Definition, characteristics, forms and surface of stems- digieduco
Types of tree forms

D. Trees-These are tall plants with a very hard and woody stem. Upto a certain height a tree remains unbranched, that portion of the plant is called trunk. The trees are usually perennial, e.g. Mangifera indica (Anacardiaceae), Polyalthia longifolia (Anonaceae), Euphorbia pulcherrima (Euphorbiaceae) etc.

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