Notes on Anatomy of Flowering Plants A tissue may be defined | Reserved
Notes on Anatomy of Flowering Plants
A tissue may be defined as, “a group of similar or dissimilar cells having common origin and performing a specific functions.”
Tissues are mainly divided into three categories:
(A) Meristematic tissues or Meristems
(B) Permanent tissue
(C) Secretory tissue
Meristematic Tissues or Meristems
(1) They contain immature and young cells and are capable of repeated divisions.
(2) Intercellular spaces are not present in meristematic tissue.
(3) They contain a homogeneous thin wall.
(4) They contain large nuclei associated with abundant cytoplasm.
(5) They are metabolically very active but they do not store food material.
(6) Only proto-plastids are present instead of plastids, chloroplast absent.
(7) Dense cytoplasm is present which contains several premature mitochondria.
(8) Vacuoles are absent.
(9) Meristematic cells are isodiametric in shape.
Types of meristems
The meristems may be classified on the basis of their mode of origin, position or function:
(i) According to origin and development: On the basis of origin, meristematic tissues are of three types :
(a) Promeristem or Primordial meristem: The promeristem originates from embryo and, therefore, called primordial or embryonic meristem. It is present in the regions where an organ or a part of plant body is initiated.
(b) Primary meristem: A primary meristem originates from promeristem and retains its meristematic activity. It is located in the apices of roots, stems and the leaf primordia.
(c) Secondary Meristem: They always arise in permanent tissues and have no typical promeristem. Some living permanent cells may regain the meristematic nature.
(ii) According to position: On the basis of their position in the plant body meristems are classified into three categories:
(a) Apical meristem: This meristem is located at the growing apices of main and lateral shoots and roots. These cells are responsible for linear growth of an organ.
(b) Intercalary meristem: These are the portions of apical meristems which are separated from the apex during the growth of axis and formation of permanent tissues. It is present mostly at the base of node (e.g., Mentha viridis-Mint), base of internode (e.g., stem of many monocots viz., Wheat, Grasses, Pteridophyts like Equisetum) or at the base of the leaf (e.g., Pinus).
(c) Lateral meristem: These meristems occur laterally in the axis, parallel to the sides of stems and roots. This meristem consists of initials which divide mainly in one plane (periclinal) and result increase in the diameter of an organ.
(iii) According to function: Haberlandt in 1890 classified the primary meristem at the apex of stem under the following three types :
(a) Protoderm: It is the outermost layer of the apical meristem which develops into the epidermis or epidermal tissue system.
(b) Procambium: It occurs inside the protoderm. Some of the cells of young growing region which by their elongation and differentiation give rise to primary vascular tissue constitute the procambium.
(c) Ground meristem: It constitutes the major part of the apical meristem develops ground tissues like hypodermis, cortex, endodermis, pericycle, pith and medullary rays.
(iv) According to plane of cell division: On the basis of their plane of cell division meristem are classified into three categories :
(a) Mass meristem: The cells divide anticlinally in all planes, so mass of cells is formed. e.g., formation of spores, cortex, pith, endosperm.
(b) Plate meristem: The cells divide anticlinally in two planes, so plate like area increased. e.g., formation of epidermis and lamina of leaves.
(c) Rib or File meristem: The cells divide anticlinally in one plane, so row or column of cells is formed. e.g,, formation of lateral root.
