SIMPLE PERMANENT TISSUE
Permanent tissues are formed of cells that have lost the power of dividing having attained their definite form and size. They may be living or dead and thin walled or thick walled. Permanent tissues are formed by differentiation of the cells of meristems (apical and lateral) and may be primary and secondary. Primary permanent tissues are derived from the apical meristems of growing regions. Primary permanent tissues may be simple and complex. A simple tissue is made up of one type of cells forming a homogenous or uniform mass. Simple tissues may parenchyma, collenchyma and sclerenchyma. Parenchyma is collection of cells which are equally expanded on all sides.
Collenchyma consists of elongated cells with the corners or intercellular spaces much thickened with deposit of cellulose and pectin.
Sclerenchyma are very long narrow thick walled and lignified cells pointed at both ends.
Stele
Vascular tissue system consists of a number of vascular bundles which are distributed in the stele. Stele is the central column of the Dicot stems and all roots surrounded by the endodermis and consist of pericycle, vascular bundles, medullary rays and pith. Each bundle may be made up of xylem tissue and phloem tissue with a cambium as in Dicot stems or without cambium as in Monocot stems or only one kind of tissue xylem or phloem as in roots. The function of this system is to conduct water and raw food material from roots to leaves and prepared food material from leaves to storage organs. Vascular bundles may be regularly arranged in a ring as in the stem of most Dicots and in all roots or they may be scattered in the ground tissue as in the stems of Monocotyledons.
ANNUAL RINGS
The activity of the cambium increases or decreases according to favourable or unfavourable climatic conditions. Thus in spring cambium becomes more active and forms a greater number of vessels with wider cavities. While in winter it becomes less active and forms elements of narrower dimensions. Wood formed in spring is called spring wood and that formed in winter is called autumn wood.
These two kinds of wood appear together as a concentric ring known as annual ring or growth ring. Successive annual rings are formed year after year by activity of cambium. Annual rings are seen by naked eye in the logs of tree trunk as in pine and timber trees. Each annual ring is one year’s growth therefore by counting total number of annual rings the age of plant can be determined.
STEM CORTEX
This is the zone that lies between epidermis and pericycle and consists of hypodermis externally, general cortex centrally and endodermis internally. Hypodermis or collenchyma lies below epidermis and has 4 to 5 layers. Genera cortex lies internal to hypodermis and consists of few layers of thin walled, large, rounded or oval parenchymatous cells. Endodermis is the internal layer of cortex consisting of more or less barrel shaped cells and surrounding the stele. Endodermis is clear outside the hard bast, but often absent on other side. It has many starch grains and is also known as starch sheath.
COMPONENTS AND FUNCTIONS OF PHLOEM
It is also called Bast. It lies towards circumference and has sieve tubes, companion cells and phloem parenchyma. Companion cells and phloem parenchyma have simple pits in the walls lying against sieve tubes. Phloem as a whole is used for translocation of prepared food material from leaves to storage organs and also to different growing regions. All elements of phloem are made of cellulose and are living. Primary phloem hardly ever contains bast fibres but it may be capped by a patch of pericycyle sclerenchyma called hard bast as in sunflower stem. Outer portion of phloem has narrow sieve tubes and is called first formed phloem or protophloem and inner portion with big sieve tubes is Metaphloem.
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