In multi cellular organisms, different parts of the body like leaves, branches, fruits, flowers carry out different functions.
However, they originate from the cells. Cells after their origin get differentiated and also associate with similar cells to carry out specific functions. A group of cells more or less alike in, form origin and development performing common functions is called a tissue.
This definition may not be quite sufficient to describe the complex tissues like xylem and phloem in higher plants. In this regard tissue can be considered from two view points.
1. From a general anatomical point of view tissue is considered as a group of contiguous cell and form a structural part of a plant.
2. From physiological point of view even widely different cells can constitute a tissue, for example, xylem consists of conducting elements, supporting elements and storage elements together forming a complex tissue.
Then a tissue may be defined as “a group of similar or dissimilar cells performing or help to perform a common function or a set of related functions, having a common origin and development.”