In this article we will discuss about the permanent tissue in plants:- 1. Simple Tissue 2. Complex Tissue.
Permanent tissues are simple or complex, according to the nature of component cells. A simple permanent tissue is composed of only one type of cells and is thus homogeneous; whereas the complex permanent tissue is heterogeneous, consisting of more than one type of cell elements.
a. Simple Tissue:
Simple permanent tissues are of following types:
(i) Parenchyma:
Parenchymatous tissue (Fig. 136) is usually made of isodiametric cells with intercellular spaces. The cells are living and contain vacuolated protoplast. The cell wall is thin, homogeneous and made up of cellulose. Only the parenchyma cells of epidermis or outermost skin of aerial organs have cuticularised outer walls.
Parenchyma is by far the most abundant type of tissue distributed in all the organs of the plants, under- ground and aerial. Their main function is the manufacture and storage of food matters, which is referred to as vital function.
Besides, the parenchyma cells may serve as storehouse of water and various waste products. They also help in the conduction of water and food matters in solution. The parenchyma cells of the epidermis are protective in function.
In the aqua tic plants the parenchyma cells often assume stellate (star-like) or armed appearance due to presence of abundant air chambers. These cells, also called aerenchyma, give buoyancy to the plants.
(ii) Collenchyma:
Collenchymatous tissue (Fig. 137) is composed of somewhat elongate cells with peculiar thickenings, confined to the corners of the cells. They often remain interlocked. The cells have protoplast. Chloroplasts are, normally absent, but may be present in some cells. They usually do not manufacture food.
Collenchyma cells look pentagonal or hexagonal in cross-section. The soft and plastic walls are made up of cellulose and pecdn. Extra-deposition of cellulose on walls abutting on intercellular spaces gives them characteristic thickened corners.
Collenchyma occurs chiefly in the superficial regions of stems (beneath the epidermis) and midribs of leaves. They are abundant in the rapidly elongating organs like leaf-stalk and floral stalk. Though soft, it is a mechanical or strength-giving tissue, particularly meant for the support of growing organs. It is called a temporary supporting tissue.
(iii) Sclerenchyma:
Sclerenchymatous tissue (Fig. 138) consists of long needle-like cells with pointed ends. As the cells are much longer than their breadth they are also called fibres. They gradually lose protoplasm and become dead. In cross-section they look typically angular. The walls are hard and lignified. Simple pits are present.
Sometimes lignification becomes so much pronounced that the central cavity (lumen) is almost blocked. They usually occur in groups or patches with peculiarly interlocked or dovetailed ends. They are present in both aerial and underground organs. Sclerenchyma is the most important mechanical or strength-giving tissue of the plant.
Sclereids or stone cells (Fig. 138) are modified sclerenchyma. They are not elongated but isodiametric or irregular in shape. The gritty texture of fruits like pears and guava, is due to the presence of sclereids. There are some peculiar types of sclereids occurring in many leaves. Ones projecting into the intercellular cavities in the stem and leaf of water-lily are hair-like in appearance; and so are also called ‘internal hairs’.
b. Complex Tissue:
A complex tissue is heterogeneous, as it consists of more than one type of cell elements. The two main complex tissues are xylem and phloem, present in the vascular bundles or conducting strands of higher plants. Vascular system is continuous in the plant body, and is instrumental in the conduction of water and soluble food materials.
(i) Xylem:
Xylem is the hard woody part of the vascular bundle which is concerned with the conduction of water. It also gives sufficient mechanical strength to the plant members.
The following elements form this complex tissue:
(i) Tracheids,
(ii) Tracheae or vessels,
(iii) Some parenchyma cells called xylem parenchyma or wood parenchyma,
(iv) Sclerenchymatous fibres called xylem fibres.
Tracheids (Fig. 139) are elongated cells with fairly large cavities and chisel-like tapering ends. They extend parallel to the long axis of the organs. Tracheids are typically angular in cross-sections. Protoplasts disappear; so tracheids are dead cells. The cell wall is hard, lignified with different types of localised thickenings, of which bordered pits are more common.
They are the primitive conducting elements, forming very important elements of xylem of the gymnosperms, but may also occur in the angiosperms. Tracheids are primarily meant for the conduction of water. Secondarily, they can give mechanical support with the aid of hard lignified walls.
Tracheae or vessels are long tube-like bodies occurring parallel to the long axis of the organs. A trachea is formed from a row of cylindrical cells, attached end to end, by the total dissolution of the partition walls (Fig. 140). So they are long tube-like bodies with fairly large cavities. Tracheae are more or less circular in cross-section.
The protoplast vanishes with the formation of the tube. The cell wall is hard and lignified with different types of localised thickenings from annular to pitted. Tracheae are the most important elements of xylem in the angiosperms, and are thus the advanced conducting elements. Their main function is conduction of water, as they are quite well-suited for the purpose. They can give mechanical strength as well.
It should be noted that a tracheid is a single cell, but a trachea or vessel is a tube-like body formed from a row of cells. The first formed xylem vessels have smaller cavities and usually annular and spiral thickenings. They are known as protoxylem vessels; but lately formed ones, called metaxylem vessels, have wider cavities and reticulate, scalariform or pitted thickenings.
(ii) Phloem:
The complex tissue phloem is composed of some thin-walled elements. It is responsible for the conduction of elaborated food materials.
The elements of phloem are mainly:
1. Sieve tubes,
2. Companion cells, and
3. Phloem parenchyma.
Often some fibres, called phloem fibres, may remain associated with them.
1. Sieve Tube:
(Figs. 141 & 142) is a long tube-like body formed by a series of cells where the partition walls are partially absorbed in a sieve-like manner. The perforated partition walls are called the sieve plates through which cytoplasm of adjoining cells are in communication.
Sieve plates may be horizontal or obliquely inclined. Unlike vessel, a sieve tube is living, having lining cytoplasm with a large central vacuole. The nucleus disintegrates with the maturity of the tube. The cell sap of the vacuoles is rich in nitrogenous matters. The wall is thin and made of cellulose. Sieve tubes are very important elements of phloem, and perform the function of conduction of prepared food materials.
A highly glistening carbohydrate, called callose, is often deposited on the sieve plate in the form of a pad known as callous pad. As a results, the cell to cell communication is temporarily or permanently cut down. The callous pad formed with the approach of winter usually dissolves in spring to re –establish the communication. This is the seasonal callus may also be formed in the functional sieves tubes.
2. Companion Cells:
(Figs. 141 & 142) are closely associated with sieve tubes and are connected with them by pores. They are elongated cells with dense cytoplasm and prominent nuclei. Elongate cells with sieve-like perforate walls are present in gymnosperms. They are called sieve cells. These are primitive, whereas the sieve-tubes are advanced elements.
Besides simple and complex tissues, there are special ones, called secretory tissues, concerned with the secretory or excretory materials of plants. Glands are the common secretory tissues which store up essential oils, etc.
Faint spots on the leaf of lemon and petals of sweetly scented flowers are the glands. Similarly ducts with resin are present in some plants. Laticiferous ducts are long tube-like bodies containing the milky fluid latex. They occur abundantly in Plumeria (B. Katgolap), Calotropis (B. Akanda).