CHAPTER XV. 



GROWTH. 



213. Definition. — ^The growth of plants is continued for a 

 much longer time than that of animals. In most cases it is 

 continued in some part throughout the existence of the plant. 

 There are also changes in the form of certain parts, particularly 

 of the lower plants, which must be distinguished from true 

 growth. Growth is a permanent change of form accompanied 

 usually by an increase in size. 



214. Formation of new parts. — Each new cell originates 

 by the division of some previously existing cell. The two 

 cells so formed grow until they attain the size of the parent 

 cell, when one or both may continue to grow until they at- 

 tain a permanent form ; then growth ceases. Those cells 

 which do not develop into permanent tissue, but retain their 

 power of division, constitute a mass of tissue at the tip of each 

 branch or root, from which all new parts regularly arise. 

 (^ 71, 87). It will be seen, therefore, that every cell of a 

 plant has been at some time in an undeveloped or embryonal 

 condition. 



215. Phases of cell growth. — The more striking charac- 

 teristics of this embryonal condition are the nearly uniform 

 and small size of the cells, and the absence or small size of 

 the water spaces (^4, fig. 117). As the cells which are des- 

 tined to become the permanent tissues grow older they pass 

 gradually from the embryonal stage into a second phase of 



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