What is Complex Tissue? Illustrate and explain a bi-collateral vascular bundle.
Primary
permanent tissue may be simple and complex. Simple tissue is made up of one
type of cells and complex tissue is made up of more than one type of cells
working together as a unit. To these may be added another kind of tissue the
secretory tissue.
Complex tissue
consists of xylem or wood which is conducting tissue and composed of elements
of different kinds like tracheids, vessels or tracheae, wood fibres and wood
parenchyma. Xylem is meant to conduct water and mineral salts from root to leaf
and give mechanical strength to the plant body. Except wood parenchyma all
other xylem elements are lignified, thick walled and dead. Phloem or bast is
another conducting tissue and is composed of sieve tubes companion cells,
Phloem parenchyma and bast fibres. Phloem is meant to conduct prepared food
material from leaf to storage organs and the growing regions.
According to
arrangement of xylem and phloem vascular bundles are Radial, Conjoint and
collateral.
Bi-collateral
vascular bundle:
When in a
collateral bundle both phloem and cambium occur twice once on outer side of
xylem and then again on its inner side. The sequence is outer phloem, outer
cambium, xylem, inner cambium and inner phloem. Bi-collateral bundle is
characteristic of the gourd family. It is always open.
In case of young
Gourd (Cucurbita) stem vascular bundles are bi-collateral usually ten in
numbers and are arranged in two ways. Each bundle consists of (a) xylem (b) two
strips of cambium and (c) two patches of phloem.
(a) Xylem occupies the centre of the bundle
and consists on outer side, of very wide vessels (pitted). Which constitute the
metaxylem and on inner side of narrower vessels which constitute protoxylem.
Protoxylem vessels remain scattered. There may be some tracheids and wood
fibres but wood parenchyma is abundant.
(b) Cambium
This tissue
occurs in two strips the outer and the inner one on each side of xylem. Its
cells are thin walled and rectangular and arranged in radial rows. Outer
cambium is many layered and is more or less flat. While the inner cambium is
few layered and curved. Each strip of cambium gradually merges into phloem and
xylem.
(c) Phloem
It occurs in tow
patches. The outer and the inner, outer phloem is plano-convex and inner one
semilunar in shape. Each patch of phloem consists of sieve tubes, companion
cells and phloem parenchyma. Sieve tubes are very conspicuous in the phloem of
the cucurbita stem. Here and there sieve plates with perforations in them may
be distinctly seen. The rest of the phloem is made up of small thin walled
cells which constitute phloem parenchyma.
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