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
Functions of Sclerenchyma:-
- Cheif mechanical tissue in mature plant organs.
- Provide rigidity to leaves.
- Provides strength to seed covering
- helps in dehiscence of seeds eg:-pods.
- Sclereids from stony endocrap of many fruits eg:- Coconut.
The apical meristem is composed of a small mass of cells which are alike and constitute the promeristem.
The cells of the promeristem differentiate into three regions, Hanstein called them histogens. Histogens
are tissue producing definte zones or regions. they are : Dermatogen, periblem and plerome, which grow
and give rise to permanent tissues. (a) Dermatogen is the single outermost layer of cells. These divide and
give rise to the skin layer (epidermis) of the stem. In roots, the cells of the dermatogens form a mass of
small tissue called calyptrogen. The calyptyogen is also meristematic and gives rise to the root cap. (b)
Periblem is located internal to the dermatogens and forms the cortex of the stem and the root. (c) Plerome
lies internal to the periblem and is the central region, where cells show a tendency to elongate. These
elongated cells form procambium that gives rise to the vascular tissues (xylem and phloem) constituting
the central cylinder or stele of the stem.
Meristem Histogen in the shoot apex (A) and root apex (B) [ longitudnal sections].
In specific areas. Depending upon thickenings it is of three types:
i) Angular Collenchyma- the thickenings are present at the angles. eg:- Stem of tagetee & Tomato.
ii) Lamellate Collenchyma- the thickenings occur at tangential walls. eg:- Stem of sunflower
iii) Lacunate Collenchyma:- The thickenings occur at the walls bordering intercellular spaces eg:-
Cucurbita stem.
Permanent Tissues
The permanent tissues comprise cells which have lost their capacity of division. The permanent tissues
may be classified into two main groups (1) simple tissues, and (2) complex tissues.
Simple tissues
These are homogeneous in nature and are composed of structurally and functionally similar cells. The
simple permanent tissues are parenchyma, collenchyma and sclerenchyma.