GENERAL CHARACTERISTICS OF GROWTH 319 



distinguish the formation of organs or differentiation as a process 

 entirely apart from growth. Although the two matters may be 

 the result of two distinct processes, increase of size and differ- 

 entiation are so closely connected that it is practically impossible 

 to separate them fully. In the following pages consequently, both 

 enlargement and differentiation will be considered but, as far as 

 possible, the two ideas will be retained distinct. 



Whatever growth may mean it does not necessarily include the 

 increase of dry weight. As a grain of corn germinates in the dark, 

 the young plantlet may reach a height of several inches. Much 

 water will be absorbed and the total weight of the plant will be 

 many times that of the dry grain. At the same time, the root, 

 stem, and leaves have developed and unfolded. Thus there has 

 been an increase of size and a rapid differentiation of parts, but 

 the plant has decreased in dry weight; no food has been manufac- 

 tured and, at the same time, respiration has been active. 



Phases of Growth. — If the cells which make up any organ are 

 studied closely, they will be seen to pass through three stages 

 of growth. The first or formative stage consists in the formation 

 of cells by cell division from previously existing ones. In the tips 

 of stems and roots are cells still capable of dividing and of form- 

 ing new cells. These growing tips are connected by a thin layer of 

 meristematic tissue called the cambium, which, like all meristem, 

 is rich in protoplasm, undifferentiated, always young, and capable 

 of producing new cells. 



The formative phase passes gradually into the second or elon- 

 gative stage, in which water enters the cells in large quantities; 

 and in the short space of a few hours, the cell may expand to 

 many times its original size. This entrance of water is due in 

 large measure to the pentoses and other " water-attracting" 

 substances inside the cell, many of which are of a colloidal nature 

 as shown by Borowikow (1912), MacDougal (1920), and others. 

 As the water enters, many vacuoles form and gradually coalesce 

 into a few large ones that fill up the center of the cell. Growth 

 is largely due, therefore, not to the formation of much new proto- 

 plasm but to the addition of water as the result of either osmosis 

 or imbibition. Although the cells of the root and stem grow in all 

 dimensions, growth is generally greater in length than in the 

 other directions and, for this reason, the period of enlargement is 

 generally spoken of as the period of elongation. 



