. What do you mean by the Tissue System and its types. Explain
The various types of tissues present in the body of a plant perform different functions. Several tissues may collectively perform the same function. A collection of tissues performing the same general function is known as a "Tissue System". There are three major tissue systems present in plants.
1. Epidermal tissue system
2. Ground or fundamental tissue system
3. Vascular or conducting tissue system
1. Epidermal tissue system: The tissues of this system originate from the outermost layer of apical meristem. It forms the outermost covering of various plant organs which remains in direct contact with the environment.
i. Epidermis: Epidermis is composed of single layer of cells.
ii. These cells vary in their shape and size and form a continuous layer interrupted by stomata. In some cases epidermis may be multilayered, e.g., Ficus, Nerium, Peperomia, Begonia etc.
iii. The epidermal cells are living, parenchymatous, and compactly arranged without intercellular spaces.
iv. Certain epidermal cells of some plants or plant parts are differentiated into variety of cell types:
a. In aerial roots, the multiple epidermal cells are modified into velamen, which absorb water from the atmosphere, e. g., Orchids.
b. Some of the cells in the leaves of grasses are comparatively very large, called bulliform or motor cells. They are hygroscopic in nature, thin-walled and contain big central vacuoles filled with water. They play an important role in the folding and unfolding of leaves. These cells develops from modification of epidermal cell and vein.
c. Some members of Gramineae and Cyperaceae possess two types of epidermal cells : the long cells and the short cells. The short cells may be cork cells or silica cells.
2. Ground or Fundamental tissue system: Ground tissue system includes all the tissues of plant body except epidermal tissue system and vascular tissues. It forms the bulk of plant body. This tissue system mainly originates from ground meristem. The ground tissue constitute the following parts :
i. Cortex: It lies between epidermis and the pericycle. The cortex is distinct in dicotyledons but not in monocotyledons, where there is no clear demarcation between cortex and pith. It is further differentiated into :
a. Hypodermis: It is collenchymatous in dicot stem and sclerenchymatous in monocot stem. It provides strength.
b. General cortex: It consists of parenchymatous cells. Its main function is storage of food.
c. Endodermis (Starch sheath): It is mostly single layered and is made up of parenchymatous, barrel shaped compactly arranged cells.
d. The inner or transverse wall of endodermal cells have Casparian strips which has deposition of suberin.
e. In roots, thick walled endodermal cells are interrupted by thin walled cells just outside the protoxylem pathches. These thin walled endodermal cells are called passage cells.
f. Endodermis with characteristic casparian bands is absent in woody dicot stem, monocot stem and leaves of angiosperms.
g. The young stems of angiosperms show a layer with abundant starch deposition. This layer occurs in the position where endodermis would have been situated which is called as starch sheath.
h. Endodermis behave as water tight dam to check the loss of water and air dam to check the entry of air in xylem elements.
i. Endodermis is internal protective tissue.
ii. Pericycle: It is a single layered or multilayered cylinder of thin-walled or thick-walled cells present between the endodermis and vascular tissues.
a. In some cases, the pericycle is made up of many layers of sclerenchymatous cells (Cucurbita stem) or in the form of alternating bands of thin-walled and thick-walled cells (Sunflower stem).
b. In monocot, the pericycle is made up of thin-walled parenchymatous cells which later on gives rise to lateral roots.
c. In dicot roots the cork cambium originates in the pericycle which results in the formation of periderm.
d. Pericycle also gives rise to a part of vascular cambium in dicot roots.
iii. Pith or medulla: It occupies the central part in dicot stem, and monocot roots.
a. It is mostly made up of parenchymatous cells.
b. In dicot roots pith is completely obliterated by the metaxylem elements.
c. In dicot stem, the pith cells between the vascular bundles become radially elongated and known as primary medullary rays or pith rays. They help in lateral translocation.
3. Vascular tissue system: The central cylinder of the shoot or root surrounded by cortex is called stele.
The varying number of vascular bundles formed inside the stele constitute vascular tissue system.
Xylem, phloem and cambium are the major parts of the vascular bundle. Vascular bundle may be of following types :
i. Radial: The xylem and phloem strands alternate with each other separated by parenchymatous cells. such kinds of vascular bundles are called radial and found mainly in roots.
ii. Conjoint: A vascular bundle having both xylem and phloem together, is called conjoint. Normally the xylem and phloem occur on the same radius