A higher plant begins its existence as a simple unicellular zygote. This zygote develops into a mature plant through a sequence of processes like repeated cell division, dif­ferentiation of cells, organization of cells into spe­cial complexes and development of organs.

Ulti­mately, the plant body gets distinguished into its organs like stem, root, leaf, flowers, fruits, etc. These organs are composed of cells which are the descendants of the zygote but the component cells are of various types, forms and remain in different groups.

The nature, structure and distribution of these cells in the organs are such that they provide mechanical strength to the organs and per­form the specific physiological functions. In lower plants like algae and fungi all cells of the plant body are alike and take part in metabolism and division.

As plants invaded land and acquired land habitat, there occurred a progressive specialization of cells accompanied with division of labour and complexities in their organization. The organs of a plant differ externally in shape as well as inter­nally in organization of component cells. This also depends on the environment in which the plant grows.