GROWTH 165 



they are not yet, nor will they ever be unless differentia- 

 tion keeps pace with growth. Does growth then consist 

 in increase in size? A cell may elongate, but this will 

 not necessarily be growth. We can easily imagine a cell 

 being subjected to a pull which would elongate it, or to 

 a pressure which would extend as well as compress it; or, 

 indeed, within the cell itself a pressure resisted less in one 

 direction than in others would cause the cell to elongate. 

 In the last case there is stretching because of the pressure 

 developed within the cell by its osmotically active contents. 

 In the two preceding cases, the cell changes its dimensions 

 because it is subjected to a force from outside itself which 

 it cannot resist. In none of these cases has growth taken 

 place. If, however, these changes in dimensions are made 

 permanent by work done by the living cell itself in translo- 

 cating and depositing insoluble material in such positions 

 as to fix the new form or the new dimensions, growth will 

 have taken place. Growth, the fixing of the new form or 

 new dimensions, follows the change accomplished from with- 

 out. But permanent change of form or of dimensions may 

 be accomplished by forces wholly outside the plant; for 

 example, by stretching the cell beyond the limit of elasticity 

 of its walls. This would not constitute growth. 



Growth, then, does not necessarily consist in an increase 

 in volume, for there are evidently cases of unquestioned 

 growth without this. The boy increasing in stature and the 

 vine increasing in length, decreasing in breadth or thickness 

 meanwhile, are growing though not increasing in volume. 

 There is necessarily increase in volume of parts or organs, 

 but not of the whole organism, in such cases of growth. 

 On the other hand a rise in turgor which increases the 

 volume of a part is not growth ; this is merely expansion. 

 Growth is a process dependent upon the formation of new 

 protoplasm, and though it usually results in increase in 

 volume, in increase in weight or mass, and in increase in 

 substance, it is not essentially any of these. 



Growth is made possible by cell-division, but it does not 

 consist in the formation of new cells, for new cells can be 

 formed by the mere division of old cells. Each cell, each 



