PLANT TISSUES

They are the tissues that are responsible for the chemical reactions and metabolisms undertaken by a plant. They are divided into different groups depending their function and origin. Plant tissues are broadly categorized into three 1. Epidermis 2. Vascular 3. Ground tissues.

Epidermis - Cells forming the outer surface of the leaves and of the young plant body.

Vascular tissue - The primary components of vascular tissue are the xylem and phloem. These transport fluid and nutrients internally.

Ground tissue - Ground tissue is less differentiated than other tissues. Ground tissue manufactures nutrients by photosynthesis and stores reserve nutrients.

Plant tissues are further divided into 1. Meristematic tissues and 2. Permanent tissues.

Meristematic tissue consists of actively dividing cells, and leads to increase in length and thickness of the plant. The primary growth of a plant occurs only in certain, specific regions, such as in the tips of stems or roots. The cells produced by this tissue are the same initially but as time moves on they become different according to where they are located. The types are

     1.  Apical Meristem - It is present at the growing tips of stems and roots and increases the length of the stem and root. They form growing parts at the apices of roots and stems and are responsible for increase in length, also called primary growth. This meristem is responsible for the linear growth of an organ.

     2. Lateral Meristem - This meristem consist of cells which mainly divide in one plane and cause the organ to increase in diameter and growth. Lateral Meristem usually occurs beneath the bark of the tree in the form of Cork Cambium and in vascular bundles of dicots in the form of vascular cambium. The activity of this cambium results in the formation of secondary growth.

     3. Intercalary Meristem - This meristem is located in between permanent tissues. It is usually present at the base of node, inter node and on leaf base. They are responsible for growth in length of the plant and increasing the size of the internodes.

Plant tissues are further classified into: 1. Simple tissues    2. Complex tissues 3. Glandular (special or secretory) tissues.

     1. Simple tissues: A group of cells which are similar in origin; similar in structure and similar in function are called simple permanent tissue. They are of four types:

     A. Parenchyma: Parenchyma (para - 'beside'; chyma - 'in filling, loose, unpacked') is the bulk of a substance. In plants. This tissue provides support to plants and also stores food. In some situations, a parenchyma contains chlorophyll and performs photosynthesis, in which case it is called a chlorenchyma. In aquatic plants, large air cavities are present in parenchyma to give support to them to float on water. Such a parenchyma type is called aerenchyma.  Prosenchyma is a type of plant tissue consisting of elongated cells with tapering ends, occurring in supporting and conducting tissue.

    B. Collenchyma is Greek word where "Collen" means gum and "enchyma" means infusion. This tissue gives a tensile strength to the plant and the cells are compactly arranged and have very little inter-cellular spaces. Collenchymatous tissue acts as a supporting tissue in stems of young plants. It provides mechanical support, elasticity, and tensile strength to the plant body. It helps in manufacturing sugar and storing it as starch. It is present in the margin of leaves and resists tearing effect of the wind.

   C. Sclerenchyma is Greek word where "Sclrenes" means hard and "enchyma" means infusion. This tissue consists of thick-walled, dead cells. Sclerenchymatous cells are closely packed without inter-cellular spaces between them. Thus, they appear as hexagonal net in transverse section. The cells are cemented with the help of lamella. Sclerenchyma cells are divided into two namely: 1. Fibres 2. Sclerides. The main function of Sclerenchymatous tissues is to give support to the plant.

    2. Complex tissues: The complex tissue consists of more than one type of cells which work together as a unit. Complex tissues help in the transportation of organic material, water and mineral up and down the plants. That is why it is also known as conducting and vascular tissue. There are two types:

    A. Xylem It carries water and dissolved substances throughout and consists of a combination of parenchyma cells, fibres, vessels, tracheids and ray cells. 

     B. Phloem is an equally important plant tissue as the xylem. Primarily, phloem carries dissolved food substances throughout the plant. This conduction system is composed of sieve-tube member and companion cells, which are without secondary walls.

DIFFERENCES BETWEEN SIMPLE AND COMPLEX TISSUES

SIMPLE TISSUES	COMPLEX TISSUES
Tissues generated from the growth of an apical meristem.	Tissues generated from the growth of a cambium.
Simple tissue consists of only one type of tissue, hence is homogeneous.	It is composed different types of cells, hence is heterogeneous.
This includes parenchyma, collenchymas and sclerenchyma.	This includes xylem and phloem.
They occur in all parts of the plant.	These have restricted distribution, they occur in vascular region only.
They are used to perform wide range of functions	Used only for conduction