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This system consists of a number of vascular bundles which are distributed in the stele. The stele is the central cylinder of the stem and the root surrounded by the endodermis. It consists of vascular bundles, pericycle, pith and medullary rays. Each bundle is made up of xylem and phloem, with a cambium in dicotyledonous stems, or without a cambium in monocotyledonous stems, or only one kind of tissue xylem or phloem, as in roots.

Function

The function of this system is to conduct water and raw food material from the roots to the leaves, and prepared food material from leaves to the storage organs and the growing regions.

The vascular bundle of a dicotyledonous stem, when fully formed, consists of three well-defined tissues: 1. Xylem or wood

2. Phloem or bast, and 3. Cambium.

[1] XYLEM

Xylem or wood is a conducting tissue and is composed of elements of different kinds, viz. (a) tracheids, (b) vessels or tracheae, (c) wood fibres and (d) wood parenchyma. Xylem, as a whole, is meant to conduct water and mineral salts upwards from the root to the leaf to give mechanical strength to the plant body.

(a) Tracheids: These are elongated, tube-like cells with

hard, thick and lignified walls and large cell cavities. Their ends are tapering, either rounded or chisel-like and less frequently, pointed. They are dead, empty cells and their walls are provided with one or more rows of bordered pits. Tracheids may also be annular, spiral, scalariform or pitted (with simple pits). In transverse section, they are angular— either polygonal or rectangular. Tracheids (and not vessels) occur alone in the wood of ferns and gymnosperms, whereas in the wood of angiosperms, they are associated with the vessels. Their walls being lignified and hard, their function is conduction of water from the root to the leaf.

(a) (b)

(b) Vessels or tracheae: Vessels are cylindrical, tube-like

structures. They are formed from a row of cells placed end to end, from which the transverse partition walls break down. A vessel or trachea is, thus, a tube-like series of cells, very much like a series of water pipes forming a pipeline. Their walls are thickened in various ways, and vessels can be annular, spiral, scalariform, reticulate, or pitted, according to the mode of thickening. Associated with the vessels are often some tracheids. Vessels and tracheids form the main elements of the wood or xylem of the vascular bundle. They serve to conduct water and mineral salts from the roots to the leaves. They are dead, thick-walled and ligni- fied, and as such, they also serve the mechanical function of strengthening the plant body.

Annular Spiral Scalariform Reticulate Vessels with simple pits Vessels with bordered pits

Fig. 4.9 Different kinds of vessels

(c) Xylem (wood) fibres: Sclerenchymatous cells associ-

ated with wood or xylem are known as wood fibres. They occur abundantly in woody dicotyledons and add to the mechanical strength of the xylem and of the plant body as a whole.

(d) Xylem (wood) parenchyma: Parenchymatous cells

are of frequent occurrence in the xylem, and are known as wood parenchyma. The cells are alive and generally thin walled. The wood parenchyma assists, directly or indirectly, in the conduction of water, upwards, through the vessels and the tracheids. It also serves to store food.

[2] PHLOEM

The phloem or bast is another conducting tissue, and is composed of the following elements: (a) sieve tubes, (b) Companion cells, (c) phloem parenchyma and (d) bast fibres (rarely). Phloem, as a whole, is meant to conduct prepared food materials from the leaf to the storage organs and growing regions.

(a) Sieve tubes: Sieve tubes are slender, tube-like struc-

tures, composed of elongated cells which are placed end to end. Their walls are thin and made of cellulose. The transverse partition walls are, however, perforated by a number of pores. The transverse wall then looks very much like a sieve, and is called the sieve plate. The sieve

plate may sometimes be formed in the side (longitudinal) wall. In some cases, the sieve plate is not transverse (hori- zontal), but inclined obliquely, and then different areas of it become perforated. A sieve plate of this nature is called a compound plate. At the close of the growing season, the sieve plate is covered by a deposit of colourless, shining substance in the form of a pad, called the callus or callus pad. This consists of carbohydrate, called callose. In winter, the callus completely clogs the pores, but in spring, when the active season begins, it gets dissolved. In old sieve tubes, the callus forms a permanent deposit. The sieve tube contains no nucleus, but has a lining layer of cytoplasm, which is continuous through the pores. Sieve tubes are used for the longitudinal transmission of prepared food materials—proteins and carbohydrates—downward from the leaves to the storage organs, and later upward from the storage organs to the growing regions. A heavy deposit of food material is found on either side of the sieve plate with a narrow median portion.

Sieve plate Sieve tube Companion cell Phloem parenchyma

Fig. 4.10 A sieve tube in longitudinal section

(b) Companion cells: Associated with each sieve lube

and connected with it by pores is a thin-walled, elongated cell known as the companion cell. It is living and contains protoplasm and an elongated nucleus. The companion cell is present only in angiosperms (both dicotyledons and monocotyledons). It assists the sieve tube in the conduc- tion of food.

(c) Phloem parenchyma: There are always some par-

enchymatous cells forming a part of the phloem in all dicotyledons, gymnosperms and ferns. The cells are living,

and often cylindrical. They store up food material and help to conduct it. Phloem parenchyma is, however, absent in most monocotyledons.

(d) Bast fibres: Sclerenchymatous cells occurring in the

phloem or bast are known as bast fibres. These are gen- erally absent in the primary but occur frequently in the secondary phloem.

[3] CAMBIUM

This is a thin strip of primary meristem lying between the xylem and phloem. It consists of one or a few layers of thin-walled and roughly rectangular cells. Although cambial cells look rectangular in transverse section, they are very elongated, often with oblique ends. They become flattened tangentially, i.e. at right angles to the radius of the stem.

Types of Vascular Bundles

According to the arrangement of xylem and phloem, the vascular bundles are of the following types:

(A) Radial vascular bundle: When the xylem and phloem

form separate bundles which lie on different radii, alternat- ing with each other, as in roots. The radial vascular bundle is the most primitive type of vascular bundles.

Phloem

ylem

(B) Conjoint vascular bundle: When the xylem and

phloem combine into one bundle, it is called as conjoint vascular bundle. There are different types of conjoint vascular bundles.

(1) Collateral: When the xylem and phloem lie together on the same radius, the xylem being internal and the phloem external is called collateral. When cambium is present in collateral as in all dicotyledonous stems, the bundle is said to be open collateral, and when the cambium is absent, it is said to be closed collateral, as in monocotyledonous stems.

Phloem ylem

Cambium

Collateral open Collateral closed

(2) Bicollateral: Phloem Cambium ylem Cambium Phloem

When the both phloem and cambium occur twice in a collateral bundle—once on the outer side of the xylem and again on the inner side of it, is called as bicollateral. The sequence is outer phloem, outer cambium, xylem, inner cambium and inner phloem. Bicollateral bundles are characteristics of Cucurbitaceae. They are also often found in Solanaceae, Apocynaceae, Convolvulaceae, Myrtaceae, etc. A bicollateral bundle is always open.

(C) Concentric vascular bundle: When one kind of

vascular tissue (xylem or phloem) is surrounded by the other is called as concentric vascular bundle. Evidently, there are two types, according to whether one is central or the other one is so. When the phloem lies in the centre and is surrounded by xylem, as in some monocotyledonous, the concentric bundle is said to be amphivasal (leptocentric). When, on the other hand, the xylem lies in the centre and is surrounded by phloem, the concentric bundle is said to be amphicribral (Hadrocentric). A concentric bundle is always closed. ylem Phloem Amphicribral (Hadrocentric) Amphivasal (Leptocentric) Phloem ylem