What is Sclerenchyma, Secondary growth, Rib meristem and Isobilateral leaf
SCLERENCHYMA
It consists of long, narrow,
thick walled and lignified cells pointed at both ends. They are fibre like in
appearance and are also called sclerenchymatous fibres or simple fibres. They
are simple often oblique, pits in their walls. The middle lamella is
conspicuous in sclerenchyma. Scelerenchymatous cells are found abundantly in
plants and occur in patches or definite layers. They are dead cells and serve
purely a mechanical function i.e. they give strength and rigidity to the plant
body and thus enable it the withstand various strains.
SECONDARY GROWTH
In study herbs and in all shrubs
and trees secondary growth takes place as a result of the formation of new
secondary tissues in them. Secondary tissues are formed by two meristems
cambium in the stellar region and cork cambium formed later in extra stellar or
cortical region. The increase in thickness due to addition of secondary tissues
cut off by the cambium and cork cambium in the stellar and extra stellar regions
respectively is spoken of as secondary growth.
Secondary growth begins with the
activity of cambium ring secondary tissues. The cambium ring as a whole begins
to cut off new cells both externally and internally. Those cut of on the outer
side are gradually modified into the elements of phloem, these constitute
secondary phloem. Secondary phloem consists of sieve tubes, companion cells and
phloem parenchyma. The new cells cut off by cambium on inner side are modified
into various elements of xylem; these form secondary xylem.
RIBMERISTEM
On the basis of plane cell
division three types of meristems are recognised mass, plate and ribmeristems.
The mass meristem shows division in all planes resulting in increase in volume
e.g. in the development of pith cortex, endosperm and sporangia. In plate meristem
the cells divide in two planes and there is increase in the area of the organ.
It results in the formation of flat structures e.g. in epidermal growth and
leaf formation. In ribmeristem the cell division is in one plane and results in
the formation of row of cells. It plays an important part in the formation of
young roots, pith and cortex of young stems in higher parts. Most of the
filamentous algae are formed due to the activity of this type of meristem.
ISO BILATERAL LEAF
It is more or less equally
illuminated on both sides. A section at right angle to one or more veins
reveals the following internal structure. The structure is more or less uniform
from one surface to the other. The epidermis on either side bears more or less
an equal number of stomata and is also somewhat uniformly thickened and cutinized.
The mesophyll is often not differentiated into palisade and spongy parenchyma
but consists of spongy cells only in which chloroplasts are evenly distributed.
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