CHAPTER XIV. 



GROWTH. 



254. 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. 



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

 in the division of some previously existing cell by a partition- 

 wall.* 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 attain 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, the primary meristem (^[ 77, 

 101). Permanent tissue which resumes active division is 

 called secondary meristem (^| 86, 134). It will be seen, 

 therefore, that every cell of a plant has been at some time in 

 an undeveloped or embryonal condition. 



256. Phases of cell development. — The characteristics of 

 this embryonal condition are the nearly uniform and small 

 size of the cells, the relatively large nuclei, and the absence 

 or small size of the vacuoles (A, fig. 181). As the cells which 

 are destined to become the permanent tissues grow older 



* To this there are only unimportant exceptions. 



178 



