Having familiarized ourselves with the different types of mechanical and conducting cells, we shall now consider the different ways in which these cells are associated to form the vascular and fibro-vascular bundles.

The simplest form of the vascular bundle occurs in petals, floral bracts, and leaves. In these parts the vascular bundle is made up of conducting cells only.

In the great majority of cases, however, the conducting cells are associated with mechanical cells to form the fibro-vascular bundle.

The fibro-vascular bundle is made up of, first, the phloem, which consists of sieve tubes, companion cells, bast fibres, and parenchyma; secondly, of the xylem, composed of vessels and tracheids, wood fibres and wood parenchyma; thirdly, of medullary rays (restricted to certain types); and fourthly, of the bundle sheath (restricted to certain types).

Types Of Fibro-Vascular Bundles

There are three well-defined types of the fibro-vascular bundle, namely, the radial, the concentric, and the collateral types.

Radial Vascular Bundles

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

In this form (Plate 114) 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.

Cross Section of a Radial Vascular Bundle of Skunk Cabbage Root.

Plate 124. Cross-Section of a Radial Vascular Bundle of Skunk Cabbage Root.

(Symplocarpus foetidus [L.], Nutt.)

1. Vessels.

2. Bundle sheath.

3. Parenchyma.

4. Sieve cells.

In sarsaparilla root, the pith is composed of thick-walled, porous pith parenchyma cells with starch. Outside the pith are arranged radial bands of oval vessels which decrease in size toward the periphery. Between the ends of these bands occur isolated groups of sieve cells.

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.

Cross Section of a Phloem Centric Bundle of Calamus.

Plate 125. Cross-Section of a Phloem-Centric Bundle of Calamus Rhizome (Acorus calamus, L.)

1. Vessels.

2. Sieve cells.

3. Phloem parenchyma.

4. Parenchyma surrounding the bundles.

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.

Cross Section of a Closed Collateral Bundle of Mandrake Stem.

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.

Bi collateral bundle of Pumpkin Stem (Curcurbita pepo, L.).

Plate 127. Bi-collateral bundle of Pumpkin Stem (Curcurbita pepo, L.).

1. Vessels.

2. Sieve tubes.

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.