282 INTRODUCTION [Ch. X. 



the constructive over the excretory processes of the plasma, or 

 from the taking in of water.* 



Of the three factors involved in growth — increase of formed 

 substance, of plasma, and of enchylema — the part played by the 

 last seems to me to have been underestimated. Plant physiol- 

 ogists have been in the best position to acquire the facts. 



trane can be permanently elongated by extension beyond the limits of elasticity 

 without the volnme necessarily increasing ; — and he apparently means to 

 include such an artificial deformation in his definition of growth. "And," he 

 continues, "under certain circumstances a diminution of volume of a plant seg- 

 ment can indeed occur as a result of growth, when, for example, the elasticity 

 of the wall is increased by growth and water is pressed from the cell until equi- 

 librium is restored." It may be doubted, however, if Pfeffer would say that 

 in this case the cell, as a whole, had grown ; but if he would, then his definition 

 is a wide departure from ordinary usage. 



Also, Vines ('86, p. 291) offers a definition, which is intermediate between 

 that of Sachs and that of Pfeffee. "By growth," he says, "we mean per- 

 manent change of form, accompanied usually by increase in bulk." But then 

 he goes on to say, "Nor does this increase even of the organized structures of 

 an organ, that is of the protoplasm and the cell wall, necessarily imply that it is 

 growing. Thus, an increase of the cell wall may take place without any appre- 

 ciable enlargement of the cell, as, for instance, when a cell wall thickens." 

 But since the thickening is a "permanent change of form," it should be consid- 

 ered by the author a growth process were not increase in size of the cell after 

 all, in the author's mind, the most important criterion of its growth. Finally, 

 Frank ('92, p. 355) finds no other criterion for growth than an increase in vol- 

 ume (dependent, however, upon the increase of a particular substance). Thus, 

 with these different plant physiologists, we see the word growth bearing the 

 ideas of increase of volume and differentiation, then of differentiation alone, 

 and, finally, of increase of volume alone. 



* Various analyses of the process of growth have been made by different 

 authors. Let us look at a few of these opinions. Says Huxley ('77, p. 2), 

 " growth is the result of a process of molecular intussusception." According to 

 N. J. C. Muller ('80, p. 100), "all phenomena of growth depend, in last analy- 

 sis, upon this, that the molecule of the solid substance is introduced into the 

 region of growth." Prank ('92, p. 355) understands by growth that increase 

 of volume which consists of the apposition or intussusception of new solid mole- 

 cules of similar matter ("welche auf der An- oder Einlagerung neuer fester 

 Molekule gleichartigen Stoffes beruhen "). According to Verworn ('95, p. 475), 

 growth is due to the excess of assimilation over disassimilation. These defi- 

 nitions include what I regard as only half the process of growth. 



On the other hand, Driesch ('94, p. 37) distinguishes two kinds of cell 

 growth: (1) passive growth, due to imbibition of water, and (2) active growth, 

 resulting from assimilation. This classification agrees with the one I have pro- 

 posed, but I think the term passive growth very inapt, since the imbibition of 

 water is as truly an active process as any other vital activity. 



