Vascular bundles are present in stele. The stele is the central column of Dicot stems and all roots surrounded by endodermis and consist of pericycle, vascular bundles, medullary rays and pith. Each bundle may be made up of both xylem tissue and phloem tissue with a cambium. Vascular bundles conduct water and raw food-material from roots to leaves and preparel food-material from leaves to storage organs and growing regions. Vascular bundles may be regularly arranged in a ring as in stems of most dicotyledons and in all roots, or they may be scattered in the ground tissue as in stems of Monocotyledons.
A vascular bundle of Dicot stem consists of three kinds of tissues (1) Xylem or wood (2) Phloem or bast and (3) Cambium.
Vascular bundles are radial when xylem and phloem form separate bundles lying on different radii alternating with each other as in roots. They are courjomt when xylem and phloem combine into one bundle. Conjomt bundles may collateral when xylem and phloem lie together on the same radius, xylem being internal and phloem external. When in a collateral bundle the cambium is present as in dicot stem, the bundle is said to be open, when cambium is absent it is said to be closed as in Monocot stems.
It is called bicollateral when in a collateral bundle phloem and cambium occur twice, once on the outer side of xylem and then again on its inner side. The sequence is outer phloem, outer cambium, xylem, inner cambium, inner phloem e.g. gourd family. It is always open.
It is called concentric when xylem lies in the centre and is surrounded by phloem as in ferms or phloem lies in the centre and is surrounded by xylem. Concentric bundle is always closed e.g. Dagger plant.
(1) Dicot Stem
T.S of dicot stem of sunflower shows Epidermis cortex and stele. In stele are vascular bundles in a ring and a large pith.
(i) Epidermis: This forms outer layer and consists of single row of cells, flattened and fitting closely along their radial walls with well defined cuticle which runs over it. It bears multicellular layers and few stomata.
(ii) Cortex: It lies between epidermis and pericycle and bears hypodermis externally, general cortex centrally and endodermis internally.
Hypodermis (collenchyma) his below epidermis and has 4 to 5 of collenchymatous cells. Cells are living and contain chloroplasts. Internal to hypodermis lays general cortex and has few layers of thin walled large, rounded or oval paranchymatous cells. Endodermis is inner most layer of cortex with barrel shaped cells surrounding the stele.
(iii) Pericycle: This is the region lying in between endodermis and vascular bundles with patches of sclernchyma and masses of parenchyma each patch is associated with phloem of vascular bundle called hard bast.
(iv) Medullary Rays: They lie in between two vascular bundles. They are few layered big polygonal or radially elongated cells.
(v) Pith: This is very large in size. It extends from below vascular bundles to the centre and is composed of rounded or polygonal thin walled living cells with conspicuous intercellular spaces between them.
(vi) Vascular bundles: They are collateral and open and are arranged in a ring. Each bundle bears phloem or bast, cambium and xylem or wood. Phloem is external and has sieve tubes which are larger elements, companion cells which smaller cells associated with sieve tubes and phloem parenchyma which is remaining mass of small cells. All above elements are thin walled and living.
Cambium is band of thin walled tissue lying between phloem and xylem. Its cells are arranged in radial rows and are rectangular in shape, very small in size and thin walled. Cambium is responsible for secondary growth.
Xylem or wood his internally and bears wood vessels which are large lignified thick walled in few radial rows. Smaller vessels lying towards the centre from protoxylem and the bigger one lying away from the centre from metaxylem.
Tracheds and wood fibres are small thick walled and lignified cells lying around meta-xylem vessels and in between them wood parenchyma is patch of thin walled parenchyma which lies on inner side of the bundle surrounding proto-xylem. Its cells are living.
(2) MONOCOT SYSTEM
T.S of Monocot stem of Maize consists of
(i) Epidermis: This is single outer most layers with thick cuticle on outer surface. Few stomata are present in epidermis.
(ii) Hypodermis (Sclerenchyma): This forms a narrow zone of sclerenchyma two or three layers thick lying below epidermis.
(iii) Ground Tissues: This is continuous mass of thin walled parenchyma extending from below the sclerenchyma to the centre. It is not differentiated into cortex, endodermis, peride etc as in Dicot stem. The cells of ground tissue enclose many intercellular spaces.
(iv) Vascular Bundles: These are collateral and closed and lie scattered in the ground tissue. They are more numerous and lie close together near periphery than the centre. Peripheral ones are also seen to be smaller in size than central ones. Each vascular bundle is oval in outline and is covered by a sheath of sclerenchyma developed on two sides upper and lower. The bundle bears xylem and phloem. Cambium is absent. Xylem consists of distinct vessels arranged in the form of Y and a small number of tracheids arranged irregularly.
Two small vessels (annual or and spiral) lying radially towards the centre form proto-xylem and two bigger vessels (pitted) lying laterally together them form meta-xylem. Besides thin walled wood (or xylem) parenchyma surround water containing cavity protoxylem and few wood fibres phloem bears sieve tubes and companion cells. No phloem parenchyma is present in Monocot stem. Outer portion of phloem which is a broken mass is protophloem and inner portion Metaphloem. The former soon gets disorganised and the latter shows distinct sieve tubes and companion cells.
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