Question

How are the vascular bundles arranged?

Answer 1

Radial Vascular Bundles

The radial type of bundle is met with most frequently in monocotyledonous roots.

In this form  the xylem forms radial bands of tissue which alternate with isolated groups of phloem. The space between the phloem and xylem is filled in with either parenchyma or fibres, or both. In some cases the vessels of the xylem meet in the centre of the root, while in other cases the centre of the stem is occupied by pith parenchyma. Each bundle is surrounded by parenchyma cells, and in iris, calamus, and veratrum, rhizomes, and endodermis, surrounds the bundles located in the centre of the stem, consisting of thin-walled (mechanical) ceils.

Surrounding the sieve cells and vessels are thick-walled, angled fibres.

External to these cells is an endodermis composed of lignified brownish-colored cells one layer in thickness.

Concentric Vascular Bundles

There are two principal types of the concentric bundle, namely, xylem-centric, in which the xylem is centric and the phloem is peripheral, as in veratrum root; and phloem-centric (Plate 125), in which the phloem is centric and the xylem peripheral, as in calamus rhizome.

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  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.

Bi-Collateral Vascular Bundles

In the bi-collateral vascular bundle (Plate 127) the xylem is in between two groups of phloem - namely, an inner group and an outer group.

In pumpkin stem a bundle occurs in each angle of the stem. The entire bundle is surrounded by parenchyma cells.

In an individual bundle the xylem consists of large circular vessels and a phloem containing large sieve cells, many of which show the yellow porous sieve plates.

Open Collateral Vascular Bundles

In the open collateral bundle (Plate 100) the xylem and phloem are separated by the cambium layer, which, through its divisions, causes the stem to increase in thickness each year. This type of bundle is characteristic of the stems and roots of dicotyledonous plants.

The bi-collateral bundle occurs in many leaves. The xylem in such cases is central, the phloem strands occupying upper and lower peripheral positions.