differentiation, in biology, series of changes that occur in cells and tissues during development, resulting in their specialization. This, in turn, permits a greater variety of organisms. In plants, unspecialized cells, composing tissue called meristem, differentiate into vascular tissue (xylem and phloem; see wood), supportive tissue (sclerenchyma), and storage tissue (parenchyma). In animals, the tissues of the gastrula stage of the embryo differentiate into specialized tissues. While it is not fully understood what initiates this processs, it is known that each of the specialized cells in an organism carries a full set of genes, with all of the organism's genetic information, but each specialized cell expresses only part of it. That is, each cell only transcribes that DNA that it needs to do its specific tasks (see nucleic acid).