In multicellular plants it seldom happens that cell division uniformly through out the whole organism. On the contrary, there are definite embryonic regions where new cells are formed by division. These regions are called ((growing points," a very inaccurate name, since they are dividing regions, the growth taking place after division' has been completed. There are two principal methods of building up a body: in one, there is a domi nant apical cell from which are cut off, the repeated division of which gives rise to the whole body; in the other, there is a group of embryonic cells (meristem), all of which divide, there being no single, dominant cell. The first type, with the single apical cell, is found in many alga, all liverworts and mosses and the ferns; the second type is found in lycopods and all the flowering plants.
Beyond the dividing region, cells grow and differentiate. Cells which are to form the conducting system elongate and secondary thickenings of their walls produce the spirals, rings, nets, pits and various markings, all of which are formed by material deposited on the inner surface of the original wall. The young cell wall consists of cellulose and most thin walled cells do not get beyond this condition; but thick walls become impregnated with vari ous substances, the most common of which is lignin, the substance which gives rigidity to all woody tissues. The characteristics of cork and bast are due to suberin; the rigidity of the stems of wheat and oats is due to silica; and there are various other constitutents of adult cell walls.
The duration of the life of a cell is .vari
ous. Some cells live only a few hours, some a few days, some live for a season and others live for years. A big tree, thousands of years old, consists almost entirely of dead cells, the life of the individual cell being comparatively brief, perhaps only a few years. The spores of the water fern, Marsilia, have germinated after resting for 50 Many seeds retain their power of germination for 20 or 30 years, some for 100 years and a few may germinate after 150 years. Tales of the germination of seeds from ancient Egyptian burial places are entirely without foundation. Seeds of corn, wheat and oats live only a few years. Since there is no cell division during the dormant period of the seed, the individual cell lives throughout this period, however long the period may be.
In Darwin's time, the great problems of biology were studied in the gross; but during the last quarter of the century there was an increasing tendency to study, both in plants and animals, the individual cell as the unit of structure and behavior. At present, the great problems of structure, development, physiology, evolution and heredity are becoming recog nized as cell problems and the increasing at tention now devoted to•cell studies promises to answer some of the most difficult questions of biology. (See PLANTS, ANATOMY OF; BLEPHARO PLAST; CYCADS ; CYTOLOGY ; HYBRIDS IN PLANTS; PLANTS, MORPHOLOGICAL EVOLUTION OF ; PLANTS, RECAPITULATION IN PLANTS, SEX IN; PLANTS, VASCULAR ANATOMY OF). Consult Hertwig, O., 'The Cell' ; Wilson, E. B., 'The Cell in Development and Inheritance.)