80 THE VITAL FUNCTIONS 



the life of the plant. In addition to this increase in 

 length of the branches, definite layers of cells inside 

 the plant may also grow, increasing the thickness of 

 the stem and root, as happens in the case of trees. 



Most animals have a- definite growth phase in youth, 

 which involves the whole of the body, but is different 

 in nature, amount and duration in the different organs. 

 Thus a child's, trunk and limbs grow more, from birth 

 to adult life, than its head, and similar differences are 

 sjiown in the growth of the different internal organs. 



Both in animals and plants it is clear that the growth 

 of an individual living cell is a different thing from the 

 growth of the organism as a whole, though the latter 

 depends upon the growth and division of the cells. 



(4) Difierentiatiou. — A multicellular organism arises, 

 during its individual lifetime, from a single cell. This 

 cell divides, and the daughter cells, instead of separating 

 from one another, as they do after cell division in 

 amoeba, remain together, and again divide, the new 

 daughter cells behaving in the same way. In this 

 manner the adult multicellular body is produced. But 

 the cells of this body are not all alike ; sets of them are 

 grouped together into tissues and organs, and the cells 

 of one tissue differ widely from those of another in 

 form, structure and function. The process of becoming 

 different is called differentiation, and accompanies the 

 growth of the bodies of all multicellular organisms. 

 Thus growth depends upon the two processes, cell 

 division and cell differentiation. 



The protoplasm of a single cell may be differentiated. 

 Thus in the Protococcus cell the chloroplast is differentiated, 

 from the colourless cytoplasm, whereas in amoeba 

 there is no such differentiation. In some unicellular 

 animals the differentiation reaches a high pitch, the 

 structure of the single cell being very comphcated. 



