Vascular Tissue System in Plants
This tissue system is composed of conducting tissues like xylem and phloem of vascular bundles.
Vascular tissues in stems and roots appear as cylinder between cortex and the pith.
In leaves, vascular tissues run parallel with the leaf surface and the corresponding veins form vascular bundles.
Each bundle is composed of two complex tissues like xylem and phloem with or without cambium (as in stems), or of xylem and phloem tissues only, as in roots and leaves.
Vascular bundle elements are derived from procambium of the apical meristem.
Type of tissue in vascular bundle
Elements of vascular bundles are (a) primary xylem or wood (b) primary phloem or bast and (c) cambium when present.
A. PRIMARY XYLEM
It contains the same basic cell types like secondary xylem i.e. tracheids, tracheae or vessels, fibres and parenchyma cells.
Developmentally primary xylem consists of first-formed protoxylem and lately-formed metaxylem.
Protoxylem differentiates in the parts of primary plant body which is still elongating and thus protoxylem is subjected to stresses, Metaxylem is not subjected to stresses as it matures after the growth or elongation of the primary body is completed.
The protoxylem contains usually tracheids, vessels and parenchyma cells ; fibres are absent.
Cells and elements of protoxylem are slender bodies with cellulose cell walls.
Annual and spiral type of thickenings due to the formation of secondary wall are the characteristic of protoxylem. Sometimes scalariform thickening may occur.
In many monocot stems, non-functioning stretched protoxylem elements are collapsed and are then occupied by open canals—the so-called protoxylem lacunae, surrounded by parenchyma cells e.g. maize (Zea mays).
Either by the determination of the position of the first-formed xylem or by the position of protoxylem in relation to metaxylem, primary xylem types may be :
When xylem develops centrifugally and the direction of protoxylem is towards the centre and that of metaxylem towards the circumference of the axis. This type is seen in stems of seed plants.
When xylem develops centripetally and the direction of protoxylem is towards the circumference and metaxylem towards the centre of the axis : this type is seen in roots of seed plants.
When maturation of cells in xylem starts in the centre and then progresses both centrifugally and centripetally—here protoxylem is either surrounded by metaxylem or flanked on two sides; seen in leaves of seed plants and some pteridophytes.
Metaxylem is more complex than the protoxylem and is composed of fibres, tracheids, vessels and parenchyma cells.
Parenchyma cells may occur in radial rows or may be dispersed among the tracheary elements.
Secondary wall thickenings are in the form of reticulate or pits only, rarely ladder-like (scalariform) thickening may be present.
The main function of xylem vessels and tracheids lies in the conduction of water and mineral salts from roots to the leaves; they also perform mechanical functions.
B. PRIMARY PHLOEM
Primary phloem is also classified into first formed elements, called protophloem and later i.e. lately-formed elements, called metaphloem.
Protophloem elements are seen in growing plant parts and are subjected to stretch and become non-functional; metaphloem elements differentiate later and constitute the only conducting phloem in mature plant parts.
Protophloem is composed of sieve elements—in angiosperms sieve tubes constitute sieve elements while in pteridophytes and gymnosperms constitutes sieve cells.
In angiosperm protophloem, sieve elements are narrow and inconspicuous and have sieve areas with callose, companion cells may or may not be present.
Sieve elements of angiosperms occur in groups or singly either among parenchyma cells or among conspicuously elongated living cells—these cells mature as fibres after the sieve elements are obliterated and thus often known as pericyclic fibres.
Metaphloem consists of much more wide sieve elements, companion cells (only in angiosperms) and parenchyma cells that become sclerified later ; fibres are generally absent.
Generally phloem occurs on the outer side of the xylem in the vascular bundles of stems and on the inner side in vascular bundles of leaves—hence phloem is called external with reference to xylem.
Sometimes phloem occurs on the inner side of the primary xylem relative to the axis of the plant—this phloem is called internal phloem as seen in Cucurbita stem.
Sometimes phloem occurs as independent strands near the pith.
Small amount of secondary phloem elements e.g. isolated sieve tubes are found embedded in secondary wood or xylem—such phloem is known as interxylary or included phloem.
The formation of interxylary phloem is the result of activity of small groups of cambium for temporary period, examples are species of Combretum, Strychnos, etc.
Cambium comprises the meristematic cells of procambium.
In gymnosperms and dicotyledons, some of the cells of procambium remain meristematic and form thin-walled rectangular cells between xylem and phloem—this is called cambium.
Cambial cells have vacuolated protoplast and thin cell wall, they divide periclinally and produce secondary tissues.
Cambium is a lateral meristem occurring parallel to the axis.
Vascular cambium is composed of two types of cells, e.g. (1) fusiform initials and (2) ray initials.
In fusiform initial, cells are spindle-shaped i.e. elongated with tapering ends; the cells of ray initials are more or less isodiametric.
D. TRANSFUSION TISSUE
In leaves of Pinus sp. (gymnosperm), vascular tissue is associated with a peculiar type of conducting tissue known as transfusion tissue.
This tissue is mainly composed of short non-living tracheids and living parenchyma cells.
The tracheids which occur next to the vascular bundle are more elongated but those occurring further away from the bundle have the shape of parenchyma cells.
The walls of tracheids are thin (though provided with secondary thickenings), slightly lignified and with bordered pits.
Protoplast is absent in the cells.
Transfusion tissue occurs adjacent to xylem at the sides of vascular bundle and may surround the bundle partly or entirely.
Albuminous cells having dense cytoplasm also occur next to the phloem within transfusion tissue.
In addition to transfusion tissue associated with vascular tissue, a so-called “accessory transfusion tissue” (Esau, 1965) is present in leaflets of Cycas,—it is composed of elongated cells (considered to be tracheids) extending from near the vein outward between the palisade and spongy parenchyma cells.
Vascular Tissue Function
The function of this tissue is still uncertain—probably it takes part in the translocation of water and food materials between the vascular bundles and mesophyll cells.