Vascular bundles are present in vascular tissue system and are distributed in the stele. The stele is the central column of Dicol stems. Each vascular bundle may be made up of both xylem tissue and phloem tissue with a cambium as in dicot stems or without cambium as in Monocot stems or of one kind of tissue xylem or phloem as in roots.
Vascular bundle may be regularly arranged in a ring as in the stems of most dicotyledons and in all roots or they may be scattered in the ground tissue as in stems of Monocotyledons.
Elements of Vascular Bundle
Vascular bundle of Dicot stem when fully formed consists of there well defined tissues (1) Xylem or wood (2) Phloem or bast (3) Cambium. They have different kinds of tissue elements.
(1) Xylem or wood
This lies towards the centre and is composed of (i) Trachae or vessels (ii) sone tracheids, (iii) a number of wood fibres and (iv) a small patch of wood parenchyma. Vessels are of various kinds such as spiral, annular, scalariform, and reticulate and pitted (with simple or bordered pits). Some tracheids also lie associated with the vessels. Wood fibres and wood parenchymatous cells lying associated with the wood or xylem provided with simple pits in their walls.
Xylem vessels and tracheids are used for conduction of water and numeral salts from roots to the leaves and other parts of the plant. Xylem parenchyma assists them in their task and also serves for food storage and wood fibres give proper rigidity to the xylem. Except wood parenchyma all other xylem elements are thick walled, lignified and dead and hence they also give mechanical strength to the plant body. The first formed xylem or Protoxylem consists of annular, spiral and scalariform vessels. It lies towards the centre of the stem and its vessels have smaller cavities. The later formed xylem or Metaxylem consists of reticulate and pitted vessels and some trachieds. It lies away from the centre and its vessels have much bigger cavities. Xylem is endarch in stems and its development is centrifugal.
(2) Phloem or Bast
This lies towards the circumference and consists of (i) Sieve tubes (2) companion cells and (3) phloem parenchyma. Companion cells and phloem parenchyma are provided with simple pits particularly in the walls lying against the sieve tubes. Phloem as a whole is used for translocation of prepared food material from the leaves to the storage organs and also to different growing regions. All the elements of phloem are made of cellulose and are living. Primary phloem hardly ever contains bast fibres but it may be capped by a patch of pericyclic sclerenchyma called hard bast as seen in the sunflower stem. The outer portion of phloem consisting of narrow sieve tubes in the first formed phloem or protophloem and the inner portion consisting of bigger sieve tubes is the later formed phloem or metaphloem.
(3) CAMBIUM
This is thin strip of primary meristem lying in between xylem and phloem. It usually consists of few layers of thin walled and roughly rectangular cells. Although central cells look rectangular in transverse section, they are much elongated often with oblique ends. They become flattened tangentially i.e. at right angles to the radius of stem. Cambium is responsible for secondary growth in thickness of plant body.
TYPES OF VASCULAR BUNDLES
According to arrangement of xylem and phloem, vascular bundles are of following types:
(1) Radial (A): When xylem and phloem form separate bundles and these lie on different radii alternating with each other as in roots.
(2) Conjoint: When xylem and phloem combine into one bundle. These are different types of conjoint bundles.
(a) Collateral (BC): When xylem and phloem lay together the same radius, xylem being internal and phloem external. When in a collateral bundle the cambium is present as in Dicot stems. The bundle is said to be open
(B) and when the cambium is absent it is said to be closed (C) as in Monocot stems (b) Bicollateral (I), when in a collateral bundle both 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 and inner phloem. Bicollateral bundle is characteristic of the gourd family. It is always open
(C) Concentric: When the xylem lies in the centre and is surrounded by phloem (E), as in ferms of phloem lies in the centre and is surrounded by xylem (I), the later type is found only in some monocotyledons e.g. sweet flag (Acorus), dragon plant (Dracaena) and dagger plant (yucca). A concentric bundle is always closed.
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