The following points highlight the three types of tissue systems in plants. The types are:- 1. Epidermal Tissue System 2. Ground Tissue System 3. Vascular Tissue System.
Type # 1. Epidermal Tissue System:
This tissue system consists of the outermost skin or epidermis of the plant organs. Epidermis is generally uniseriate or single- layered and protective in function. Many-layered epidermis, called multiple epidermis, is present in the leaves of banyan and roots of orchids. Epidermal cells are of modified parenchymatous type. In surface view, they look irregular in shape.
They are compactly arranged without leaving intercellular spaces. The cells are living with peripheral cytoplasm, round central vacuole, nucleus and minute leucoplasts.
Chloroplasts are present only in the epidermal cells of aquatic and shade plants and guard cells of the stomata. In aerial organs the outer wall is cuticularised for checking the loss of water. Besides that, deposition of waxy or resinous matters may take place on the surface.
Many outgrowths, mostly multicellular and rarely unicellular, may develop on the epidermis of stems and leaves. The epidermis of root has no cutinised outer wall and bears unicellular hairs at the root-hair zone. Root-hairs are, of course, prolongations of the cells themselves. The epidermis of root is also called epiblema or piliferous layer (Fig. 153).
Epidermis is a continuous layer but for some minute openings, called stomata (singular, stoma) present in the aerial parts of the plants. Every stoma has a small opening called stomatal aperture and two crescent-shaped cells known as guard cells (Fig. 143).
Guard cells have dense cytoplasm, nucleus, chloroplastids and even starch grains. Semilunar protoplasmic guard cells can be very easily distinguished from vacuolated irregular epidermal cells.
Stomata may be conveniently observed in surface view by peeling off the epidermis of the leaf. Stoma is formed from a mother cell which splits lengthwise into two parts leaving a small slit between them.
The slit develops into stomatal aperture and the two cells into two guard cells. The aperture opens into a cavity, called sub-stomatal chamber, which, is in communication with the intercellular space system of the internal cells (Fig. 144).
The guard cells regulate the opening and closing of the stomata which usually remain open in day time and close up with the nightfall. In day time the guard cells manufacture sugar. The concentration of sap increases and water is absorbed from the neighbouring epidermal cells by osmosis.
The guard cells thus become turgid or full of water, then they bulge out in outward direction making the aperture wide open. At night there is no manufacture of sugar, concentration of sap decreases. The guard cells lose water and become flaccid. Then they lie side by side, so that the aperture is closed.
Stomata are present in green aerial organs of the plants, particularly in the green leaves. Dorsiventral leaves have stomata on the lower surface, isobilateral and centric leaves have evenly distributed stomata; and in floating leaves they are on the upper surface.
In an individual leaf there are more numerous stomata near the apex, whereas minimum number is found near the base, the middle portion having a distribution which is an average of the base and the apex.
Stomata usually occur in parallel rows in the leaves of monocotyledons, and he scattered in the reticulately-veined leaves of the dicotyledons. Their number is fairly large which may range between a few thousand to over a hundred thousand per cm.
Stomata carry on the gaseous interchange between the internal tissues and the outer atmosphere. Normally the guard cells remain in the same level with adjacent epidermal cells. But in plants growing in dry situation the stomata are located at the bottom of a depression. They are known as sunken stomata, which can considerably reduce transpiration.
Type # 2. Ground Tissue System:
This tissue system is heterogeneous consisting of different types of tissues, of which parenchyma is most abundant. It extends from beneath the epidermis right up to the centre, thus covering the major portion of the organs. In stems and roots ground tissue system is differentiated into two zones—extra-stelar or tissues lying outside the stele and intra-stelar or those present inside the stele.
Stele is the central cylinder or column of the organ which includes the vascular bundles. Extra-stelar ground tissue, known as cortex, is often differentiated into three zones, viz. hypodermis, parenchyma and starch sheath or endodermis; and intra-stelar ground tissues into pericycle, medullary rays and medulla or pith.
In leaf the ground tissue goes by the name of mesophyll tissue, usually differentiated into palisade and spongy cells. The ground tissues will be discussed in detail along with primary structures of the organs.
Type # 3. Vascular Tissue System:
It consists of the vascular bundles or conducting strands present in the different plant organs. This tissue system is continuous in roots, stems and leaves. A vascular bundle is composed of two complex tissues—xylem and phloem, also called hadrome and leptome respectively.
In stems of dicotyledons and gymnosperms a strip of lateral meristem, called cambium, is present between xylem and phloem, and this is responsible for growth in thickness of the stems. If cambium is present, the bundle is called open; if absent, the bundle is said to be closed.
According to the position of xylem and phloem, the vascular bundles are of the following types (Fig. 145):
(a) Radial:
Here xylem and phloem occur as separate patches and remain arranged on alternate radii. Radial vascular bundles found only in roots.
(b) Collateral:
In this type xylem and phloem together form a bundle and thus lie on the same radius, xylem is internal, i.e. towards centre, and phloem is external. Collateral bundle may be open, as in many dicotyledonous stems, or closed, as in monocotyledonous stems.
(c) Bicollateral:
It is a collateral bundle where xylem is in the centre, two strips of cambium, outer and inner, and two patches of phloem, outer and inner, occur on the two sides of xylem. Thus the sequence is outer phloem, outer cambium, xylem, inner cambium and inner phloem. This type of bundle is present in Cucurbita family.
(d) Concentric:
Here one type of tissue surrounds the other. If xylem is in the centre surrounded by phloem, the bundle is called hadrocentric or amphicribral, as in fern; and when phloem is surrounded by xylem, the bundle is leptocentric or amphivasal, e.g. Dracaena. Concentric bundles are always closed.