PinkMonkey Online Study Guide-Biology
Evolution of vascular tissues in plants was one of the
essential adaptations to terrestrial habitats. Vascular tissues are the
specialized conducting tissues, xylem and phloem. Plants
with vascular tissues are grouped as the Tracheophyta and include
Pteridophyta, Gymnosperms and Angiosperms.
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Xylem: It is a specialized tissue for conduction
of water and minerals from roots to all other parts of the
plant. Xylem is a complex and heterogeneous tissue composed of four types
of elements namely (a) tracheids ,(b) vessels ,(c) fibers
,and (d) xylem parenchyma (Fig. 15.24A). The first three
elements are dead and lignified. Hence, xylem is described
as a dead tissue.
Phloem: It is a living, complex and heterogenous
food-conducting tissue. It is composed of (a) sieve tubes, (b)companion
cells, (c) phloem fibers and (d) parenchyma (Fig. 15.24B).
In plants, vascular tissues are organized into strands called vascular bundles. These are spread over all parts of the plant and form the vascular tissue system. In gymnosperms and dicotyledons, cambium is also present in the vascular bundles.
Steles: The vascular tissues are organized into
steles. The vascular bundles, with or without other non- vascular
tissues, when surrounded by the pericycle and the endodermis are referred
to as steles.
Main types of steles and their evolution
i) Protostele (Fig 15.25A): This is the simplest
and most primitive type of stele in which xylem forms the central cylinder
and is completely surrounded by phloem cells. Pith is totally absent.
Common in pteridophytes like Psilotum, Lycopodium, etc.