Tissues : A tissue is a group or collection of similar cells, which work together to achieve a particular function and have a common origin. Blood, phloem and muscles are examples of tissues.
Simple tissue : These are made up of only one type of cells. This type of tissue is similar in structure and function.
Simple tissues are further classified into three types :
(1) Parenchyma
(2) Collenchyma
(3) Sclerenchyma

(1) Parenchyma : These are the simplest form of tissues with little specialisation. They have relatively unspecialised cells, with thin cell walls. They are loosely packed with inter-cellular spaces. This tissue provides support to plant, store food and allows the exchange of gases.
(2) Collenchyma : Collenchyma, being a strong and flexible tissue, has living and thick - walled cells; thickenings are present at the corners of the cells and contain cellulose and pectin; lignin is never present. Intercellular spaces are absent. Cells may be circular, oval or polygonal in shape.
A few chloroplasts may be present in the cells. Collenchyma is present in the peripheral regions of stem and leaves. Collenchyma is mainly a mechanical tissue and provides mechanical strength and elasticity to the growing stems.
(3) Sclerenchyma : Sclerenchyma is a strengthening tissue. Cells are dead and possess hard, rigid, very thick lignified walls; lignin is a waterproof material. Intercellular spaces are absent. Sclerenchyma cells are of two types - fibres which are long, narrow, pointed cells, and sclereids which are shorter, isodiametric or irregular cells; sclereids are also called stone cells or grit cells. The walls of sclerenchyma cells contain oblique thin areas, called pits. Sclerenchyma is a mechanical and protective tissue.