Shoot system in Plants: Morphology of Flowering Plants I Plant Anatomy #biology #pgtbiology #kvs #nvs #emrs

 STRUCTURE

                                

Shoot Tip. The stem shoot apex produces leaf primordia that develop into leaves.

Node. The site where a leaf as well as axillary buds, arise on the stem is termed a node

Internode. The elongated portion of a stem between two nodes is called an internode

Young stem comprises of three main regions

A terminal bud which develops at the apical end of the stem, encloses a meristematic stem axis with its apical meristem

Axillary buds are lateral buds that form at the leaf-stem axils (nodes)

Bud will give rise to flower or branches.

Leaf Scar. A scar remains on the stem where a leaf stalk (petiole) was attached. Leaf scars and buds are species-specific, and therefore, are useful for winter plant identification.

Dicot stem comprises of following layers:

Epidermis

Cortex

Stele

EPIDERMIS:

The epidermis consists of a single layer of cells and is the outermost layer of the stem. It contains stomata and produces various types of trichomes. The outer cell walls are greatly thickened and heavily cutinized. The cells are compactly arranged and do not possess intercellular spaces. It serves mainly for restricting the rate of transpiration and for protecting the underlying tissues from mechanical injury and disease-producing organisms.

CORTEX:

The region that lies next to the epidermis is the cortex. The innermost layer of the cortex is the endodermis, also known as the starch sheath. It consists of a single layer of cells which surrounds the stele and contains numerous starch grains.

The part of the cortex situated between the epidermis and the endodermis is generally divided into two regions-an outer zone of collenchyma cells and an inner zone of parenchyma cells.

Collenchyma

On the inner side of the epidermis, there is usually a band of collenchyma. The cells of the collenchyma are modified parenchyma cells with cellulose walls thickened at the angles where three or more cells are in contact. The cells are alive and in having a moderate amount of protoplasm.

The chief function of collenchyma cells is to serve as strengthening material in succulent organs which do not develop much woody tissue, or in the soft young parts of woody plants before stronger tissues have been developed.

Parenchyma

The parenchyma cells are generally regular in shape, have thin walls. They are living cells. When they are exposed to the light they develop chloroplasts and are known as chlorenchyma cells. Chlorenchyma cells are thus only a special kind of parenchyma cells. The turgid parenchyma cells frequently help in giving rigidity to an organ. The function of parenchyma cells is important in succulent stems and in the young parts of the stems and woody plants before strong mechanical tissues have been developed.

Sclerenchyma

The sclerenchyma cells are found in the cortex of some stems. There are two varieties of these sclerenchyma cells-short or irregularly shaped cells, known as stone cells, and sclerenchyma fibres, that are long, thick-walled dead cells and serve as strengthening material. Stone cells give stiffness to the cortex. The sclereids have been reported from the cortex of many water plants (e.g. Limnanthemum, Nymphaea, etc.).

Endodermis

The innermost layer of the cortex is the endodermis consisting of barrel-shaped, elongated, compact cells, having no intercellular spaces and contain starch grains and, thus, the endodermis may be termed starch sheath. It consists of a single layer of cells which surrounds the stele and contains numerous starch grains.

Stele

The part of the stem inside of the cortex is known as the stele. The stele consists of three general regions pericycle, vascular bundle and pith.

Pericycle

The region between the vascular bundles and the cortex is known as the pericycle. It is generally composed of parenchyma and sclerenchyma cells; sclerenchyma cells may be absent. The sclerenchyma may occur as separate patches or as a continuous ring in the outer part of the pericycle, forming a sharp line of demarcation between the stele and the cortex. The sclerenchyma cells are long, thick-walled dead cells, which serve as strengthening material.

Vascular Bundles: Vascular Bundles are Open, shows endarch pattern of development and organization of stele is Eustele. In eustele the vascular tissue is arranged in discrete vascular bundles, usually in one or two rings around the central pith



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