BOTANY Higher Secondary Second Year - Textbooks Online

Collenchyma may contain chloroplasts and carry out photosynthesis.
Collenchyma is divided into three types – lamellar, angular and lacunate
collenchyma.
In the hypodermis of Helianthus, only the tangential walls of
collenchyma are thickened and the radial walls are devoid of thickening.
This type of collenchyma is called lamellar collenchyma. In the
hypodermis of Datura and Nicotiana, the cell walls of collenchyma are
thickened at their angles. This type is called angular collenchyma. In the
hypodermis of Ipomoea, the cell wall thickening materials are deposited
on the walls bordering the intercellular spaces. This type is called lacunate
collenchyma.
Sclerenchyma
Sclerenchyma is a dead tissue. The cells have lignified secondary
walls. They lack protoplasts. On the basis of origin, structure and
function, sclerenchyma is divided into two types – sclereids and fibres.
The sclereids are different from fibres in the following respects. Sclereids
are shorter whereas fibres are longer. Sclereids possess numerous pits
as compared to the fibres.
Sclereids
Sclereids are dead cells. They vary greatly in shape and thickness.
The cell wall is very thick due to lignification. Lumen is very much
reduced. The pits may be simple or branched. Usually sclereids are
isodiametric, but in some plants they are elongated. They are responsible
for the rigidity of the seed-coat. The isodiametric sclereids are called
brachy-sclereids (stone cells). They are found in bark, pith, cortex, hard
endocarp and fleshy portions of some fruits. eg. pulp of Pyrus.
Elongated rod shaped sclereids are called macrosclereids (rod cells).
They are found in the outer seed coat. eg. Crotalaria. The rod shaped
sclereids with dilated ends are called osteosclereids (bone cells). eg. seed
coat of Pisum.
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