The fundamental unit of a living organism and the basis of its structure and physiology. A living plant cell comprises *cell wall
. Bacterial cells and those of the blue-green algae are *prokaryotic
, i.e. they have no nucleus. The cells of all other organisms are *eukaryotic and have a well defined nucleus containing the chromosomes enclosed in a nuclear membrane. In many plant tissues, e.g. sclerenchyma, cells are in a nonliving state, consisting of cell wall only or cell wall thickened with additional nonliving material such as lignin.