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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