A group of cells, that are similar in structure and work together to achieve a particular function, forms a tissue. There are two types of tissues. They are complex tissues and complex issues. Simple permanent can be categorized into parenchyma, collenchyma and sclerenchyma based on their function. Complex permanent tissue comprises of xylem and phloem. Xylem is useful in the transport of water and soluble substances. Phloem is useful in the transport of food molecules.