Depending upon thickenings , There are of three
types:
i) Angular Collenchyma:- The Thickenings are
present at the angles.
eg:- Stem of Tagetes & Tomato.
ii) Lamellate Collenchyma:- The thickenings
occur at tangential walls. eg:- Stem of Sunflower.
iii) Lacunate Collenchyma:- The thickenings
occur at the walls bordering inter cellular spaces
eg:- Cucurbita stem.
Sclerenchyma: This tissue differs from previous
tissues in possessing thickened secondary walls
which may or may not be lignified. They are of
various shapes and sizes when mature. They are
dead.
They are of two types :- Sclerenchymatous fibres
- Sclereids
- Sclerenchymatous fibres
Fully developed fibres are long and tapering at
the ends. In some cases the cellwall becomes so
much lignified that the lumen is greatly reduced.
Wall has small circular pits. The normal length
of fibres is 1-3 mm but in jute and Boehmeria
their length may be from 20-550 mm.The fibres
are present in hypodermis of monocot stem,in
pericycle of many dicots, in secondary wood,
in vascular bundle sheath in monocot stems and
hypodermis of many leaves. Commercial fibres
are obtained from jute, Hemp, Flax - Sclereids:- They are highly Thickened dead
lignified sclerenchyma cells with very narrow
cavities. Different type of sclerids are
i) Brachysclereids / Stone cells:- They are short,
unbranched, isodiametric with branched pits.
eg:-girth of guva, sapota, peas etc.
ii) Macrosclereids:- Elongated & rod-like, eg:-
epidermal covering of legume seeds.
iii) Osteosclerids:- Bone-like with swollen ends.
eg:- sub-epidermal covering of some legume
seeds
iv) Astrosclereids:- Branched like star. eg:- tea
leaves, petiole of lotus.
v) Filiform sclereids:- Fibre-like eg:- olea.
vi) Trichosclereids:- elongated hair-like
Parenchyma: Cells are isodiametric and thin walled.
The cells may be oval, circular or polygonal with
intercellular spaces. The cell-wall is made up of
cellulose.They are living. This tissue is present in
all organs of the plant e.g., roots, stems, leaves,
flowers, fruits and seeds. The cells of parenchyma
are involved in the various physiological activities
like photosynthesis, assimilation, storage,
secretion, excretion, etc.
It is modified variously to perform special
functions.
a) Fibre-like elongated parenchyma- Prosenchyma.
It is thick walled & provide rigidity& strength.
b) Cutinised parenchymatous cells from epidermis. It
is protective layer. cuticle reduces transipiration.
c) The protective parenchyma without Cuticle in the
root forms epiblema.
d) xylem parenchyma & phloem parenchyma -
storage of food & conduction of water and in later
storage & lateral conduction of food.
e) Parenchyma cells containing chloroplast -
Chlorenchyma. forms mesophyll of leaves.
f) In aquatic plants & some land plants air filled
parenchyma- Aerenchyma - helps in buoyancy.
g) Idioblasts - There are non green large-sized
parenchyma cells which possess inclusions like
tannins, oil etc.
h) Secretory cells:- are parenchyma cells that
produce nectar, oil etc.
Collenchyma:Collenchymatous cells are longer
than parenchyma cell.. They are living. The
intercellular spaces in this tissue are absent
because in intercellular spaces at the corner of
cells thickenings of cellulose and pectin develop
due to which the cell wall become rigid and thick
prevents teasing of leaves, its cells store food.
Sometimes collenchyma develops chloroplasts.
Collenchyma is usually found as 3-4 layered
hypodermis of herbaceous dicotyledonous stem.
They are absent in monocot stems. Collenchyma
provides mechanical support to the organs and due
to its peripheral position in stems it resists bending
and pulling action of wind. When chloroplasts are
present it takes part in photosynthesis.