(I) PRIMARY STRUCTURE OF PLANTS
PLANT ANATOMY ::
¶ It is the branch of Botany which deals with study of internal structures and organization of plants by the section cutting is called Plant anatomy.
¶ Anatomy is a Greek Word. Ana → asunder & temnein → to cut. Plant anatomy is also called as Internal Morphology.
¶ N.Grew is known as father of plant anatomy.
¶ K.A. Chaudhary is known as father of Indian plant Anatomy.
PLANT TISSUE ::
¶ An organized group of cells which is having similar or dissimilar in shape, having a common origin and usually performing a common function is called tissue.
¶ The term tissue was coined by Nehemiah Grew.
MERISTEMATIC TISSUE :
¶ Term given by Nageli.
¶ Meristem : Growth in plants is largely restricted to specialised regions of active cell division called meristerm./A meristem is a localized region in which actual cell division occurs.
CHARACTERISTICS OF MERISTEMATIC TISSUES ::
¶ It is an undifferentiated tissue.
¶ Cell cycle of meristem is in continuous state of division. It means they have the capacity to divide. So meristematic tissue is composed of immature cells.
¶ Meristematic cells have only primary cell wall which is thin and flexible (elastic) and made up of cellulose. Secondary cell wall is absent.
¶ Cells of meristem are small and isodiametric.
¶ They have dense cytoplasm.
¶ Normally vacuoles are absent in meristematic cells but if present then small.
¶ They have prominent and large nucleus.
¶ Meristematic cells are metabolically highly active so lack of reserve food occur in these cells.
¶ Plastids are absent in meristems. If they are present, then only in the proplastid stage ER is poorly developed.
¶ They do not have intercellular spaces. Cells are closely fitted (Packed) together. So it is a compact tissue.
CLASSIFICATION OF MERISTEMATIC TISSUE :
[A] MERISTEMATIC TISSUE BASED ON ORIGIN AND DEVELOPMENT ::
On the basis of origin and development meristems can be divided into following three types :
(i) Promeristem/Embryonic Meristem/Primordial Meristem :
¶ This meristem develops in begining during embryonic stage.
¶ They divide and give rise to primary meristem.
(ii) Primary meristem :
¶ Meristematic cell developed from promeristem are known as primary meristem.
¶ These cells are always in division phase and form primary permanent tissue.
¶ They are present below the promeristem at shoot and root apices, at the apex of leaves and in intercalary parts.
(iii) Secondary meristem :
¶ These are the meristems developed from primary permanent tissues. They are not present in the embryonic stage of the plant. These are present in mature region of root and stem of many plants particularly those that produce woody axis.
¶ Some of the cells of primary permanent tissues become meristematic and constitute secondary meristem.
¶ By the activity of secondary meristems, secondary growth takes place.
¶ Cork cambium, Interfascicular cambium & root cambium are excellent examples of secondary meristems.
Note : Formation of meristem from any permanent tissue is called dedifferentiation.
or
Formation of undifferentiated tissue from differentiated tissues is called dedifferentiation.
[B] MERISTEMATIC TISSUES BASED ON LOCATION (POSITION) IN PLANT BODY ::
¶ On the basis of position, meristematic tissues are divided into three types :
(i) Apical Meristem :
¶ The meristems which occur at the tips of roots and shoots and produce primary tissues are called apical meristems. They are responsible for increase in the length of plant organs.
Example : Root apex, Shoot apex. They are responsible for primary growth
¶ During formation of leaves and elongation of stem, some cells left behind, they form auxillary bud and from new branches or a flowers.
¶ Apical meristem in shoot and root is terminal and subterminal respectively.
(ii) Intercalary Meristem :
¶ The meristem which occurs between mature tissues.
¶ This is the separated region of apical meristem.
¶ By the activity of this meristem length of the plant organs increases.
¶ They are present in some plants stem.
¶ They are responsible for regeneration of parts removed by grazing herbivores in grasses.
¶ They may be present either at the base of internode e.g., grasses, bamboo and Equisetum etc. or at the base of node e.g., Mint. They are also present at the base of leaves e.g., Pinus. By the activity of this meristem, length of leaves increases.
Note : They are short lived and convert into permanent tissue. Both apical meristem & intercalary meristems are primary meristem because they appear early in the life of a plant and contribute to the formation of primary plant body.
(iii) Lateral Meristem :
¶ Lateral meristem occurs in lateral side of plant organs or parallel to the longitudinal axis (Tangential plane) of plant organs. They are cyllindrical meristem.
¶ Activity of lateral meristem increases the girth of plant organ, so it is responsible for secondary growth and produce secondary tissue.
¶ Lateral meristems are both primary and secondary in origin (mostly secondary in origin). There are two examples of primary lateral meristem.
1. Marginal meristem :- It occurs at the margin of leaf. Its activity increases the width of leaf so total growth of leaf is called intercalary marginal growth.
2. Intra fascicular cambium or fascicular cambium :- This cambium occurs inside the vascular bundle of the stem. Except intra fascicular cambium all cambia are secondary in origin.
[C] CLASSIFICATION BASED ON PLANE OF DIVISION ::
(i) Rib-Meristem/File Meristem :
• Meristem in which anticlinal division occurs in one plane. For example, tunica is a type of rib-meristem. Formation of some cells of cortex and pith takes place by this meristem.
(ii) Plate-Meristem :
• Meristem which divides anticlinally into two plane at right angle to each other. By this division a plate like structure is formed. Formation of leaf blade takes place by the activity of this meristem.
(ii) Mass-Meristem :
• Meristem which divides in all possible planes resulting in the increase in the volume of plant body (organ). Example : The formation of embryo and endosperm takes place by this kind of meristem.
¶According to Foster, meristem is classified into two region on the basis of rate of division : -
(i) Summit (ii) Flank
(i) Summit : The rate of division is slow in this region. This region is located at the apex.
(ii) Flank : The rate of division is very fast in this region. This region lies behind the summit and leaf primordia are formed by this region.
• Time period between initiations of two successive leaf primordia is called "Plastochron".
• Growth of leaf primordium is → First apical then marginal.
• During reproductive phase i.e., at the time of flowering, vegetative shoot apex transforms into reproductive shoot apex. This change in shoot apex is induced by florigen & light.
• Summit zone of reproductive shoot apex is more active i.e., rate of cell division is greater and it forms stamens & Carpels and flank zone is less active in reproductive shoot apex and it forms sepals and petals.
