Describe internal structure of cucubita stem (or young gourd stem)


It bears epidermis, cortex and stele. There are five ridges, five rows, ten vascular bundles in two rows, the outer row to the furrows and the central cavity (stem being hollow).

(1)        EPIDERMIS
This is single outermost layer passing over the ridges and furrows. It often bears many long and narrow multicellular hairs.

(2)        CORTEX
This consists of epidermis externally, general cortex in the middle and endodermis internally. Hypodermis (collenchyma) lies immediately below epidermis and consists of six or seven layers pf collenchymatous cells in the ridge. In the furrows the number of layers in reduced to two or three sometimes none. In the furrows the under dying parenchyma may be seen to pas right upto the epidermis. Collenchyma cortex forms a narrow zone of parenchyma 2 or 3 layers thick.

In the furrows it often passes outwards right upto the epidermis. Chloroplasts are abundant in the cortex. Endodermis is the innermost layer of the cortex lying outside the pericycle. This layer is away in outline and contains starch grains.

(3)        PERICYCLE
Below epidermis is a zone schlerenchyma which represents the pericycle. This zone consists of 4 or 5 layers of thick walled lignified cells which are polygonal in shape.

(4)        GROUND TISSUE
This is continuous mass of thin walled parenchyma extending from below the sclerenchyma to the pith cavity. In this tissue lie embedded vascular bundles.

(5)        VASCULAR BUNDLES
These are bi-collateral, ten in number and are arranged in to rows. Each bundle consists of (a) xylem (b) two strips of cambium and (c) two patches of phloem.

(a) Xylem: It occupies the centre of the bundle and consist on the outer side very wide vessels (pits) which constitute metaxylem and on inner side narrower vessels which form the protoxylem. Protoxylem vessels remain scattered. There may be some tracheids and wood fibres but wood parenchyma is abundant.

(b) Cambium: This tissue occurs in two strips the outer and the inner one on each side of xylem. Its cells are thin walled and rectangular and arranged in radial rows. Outer cambium is many layered and is more or less flat while inner cambium is few layered and curved. Each strip of cambium gradually merges into phloem and xylem.

(c) Phloem: It occurs in two patches the outer and the inner. Outer phloem is plano-convex and the inner one semilunar in shape. Each patch of phloem consists of sieve tubes, companion cells and phloem parenchyma. Sieve tubes are very conspicuous in the phloem of cucurbita stem. Here and there sieve plates with perforations in them may be distinctly seen. The rest of the phloem is made up of small thin walled cells which constitute the phloem parenchyma.

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