Question

Comment on structure, type and functions 

(a) Parenchyma

(b) Sclerenchyma

Answer 1

(i) Parenchyma:
Parenchyma (Greek, para = beside; en – chein = to pour) is most simple and unspecialized tissue which is concmed mainly with the ordinary vegetative activities of the plant. It is phylogenetically and ontogenctically the primitive tissue. The tissue mainly consists of thin walled living cells which have intercellular spaces between them The cell wall is made of cellulose or calcium pectate.

Possesses a prominent nucleus and vacuolate cytoplasm The intercellular spaces are either formed schizogenously (i.e by splitting apart of the middle lamellae region between cells) or lysigenously. However, they remain attached by plasmodesmata. Sometimes the intercellular spaces are absent. Each individual parenchymatous cell may be spherical, oval, cylindrical, rectangular, stellate or long spindle like. It is that cells varies in different plants and in different organs of the same plant.

Parenchyma is distributed in almost all the parts of plant body constitutiong the vegetative ground tissue. It is found in epidermis, cortex, pith, pericycle, mesophyll of leaves, pulp of fruits, endosperm of seeds and in meristematic tissues. Parenchymatous cells are also found in xylem and phloem.

Functions of Parenchyma :

Parenchymatous tissue performs the following functions in different plant organs –

1. Storage of reserve food materials.
2. Storage of water in succulents, (e.g., Opuntia, Euphorbia)
3.  Buoyancy and gaseous exchange in water plants. (Aerenchyma).
4. Provide rigidity to plant body due to turgidity and help to maintain the shape of plant body.
5. They give rise to secondary meristem in the form of cork cambium and vasular cambium, (secondary growth and healing)
6. They perform all vital activities of plants.
7. Sometimes develop chloroplasts and called as chlorenchyma and take part in photosynthesis.

(ii) Sclerenchyma:
Sclerenchyma (Greek, sclerous = hard; enchyma = an infusion) consists of thick – walled dead cells.Cells vary in shape, size and origin.They possess hard and extremely thick secondary walls due to uniform deposition of lignin . Sometimes, they may be unlignified. In the beginning the cells are living and have protoplasm, but due to deposition of impermeable secondary walls they become dead.

(b) Sclereids :
The sclerenchymatous cells, which are short and possess extremely thick lamellated lignified walls with long tubular simple pits are called sclereids. They originate from ordinary parenchymatous cells by the deposition of secondary wall layers. They may be simple or branched. The sclereids vary greatly in their shape and size, they may be spherical, oval, cylindrical, T shaped, dumbell – shaped or even stellate. They are usually shorter than fibers.