COLLATERAL VASCULAR BUNDLES
There are three types of collateral vascular bundles—namely, closed collateral, bi-collateral, and open collateral.
In the closed collateral bundle the phloem and xylem are not separated by a cambium layer, and in many cases the bundle is surrounded by thick, angled walled fibres, as in palm stem. The term closed bundle refers to the fact that there is no cambium between the xylem and phloem, therefore the bundle is “closed” to further growth, and not to the fact that it is frequently surrounded by fibres which prevent further growth. In podophyllum stem (Plate 126) the xylem portion of the bundle faces the centre of the stem and the phloem portion of the bundle faces the epidermis. The xylem and phloem are separated by a cambium layer, and both are surrounded by thick-walled angled fibres which are the chief mechanical cells of the stem. This bundle is, in fact, mechanically closed, but not physiologically because a cambium is present.
PLATE 126
Cross-Section of a Closed Collateral Bundle of Mandrake Stem
(Podophyllum peltatum, L.)
1. Vessels.
2. Sieve cells.
3. Cambium.
4. Fibres.
5. Parenchyma.
6. Intercellular space.
PLATE 127
Bi-collateral bundle of Pumpkin Stem (Curcurbita pepo, L.)
1. Vessels.
2. Sieve tubes.