short notes on Simple permanent tissue, Stele, Annual rings, Stem Cortex and Components and functions of Phloem
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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