54] CHAP. IV. SPECIAL PHYSIOLOGY OF REPRODUCTION. 227 



ternal resistance, as when the roots of tree cause the splitting of 

 walls or of pavements. Although one essential factor in turgidity 

 (see p. 159) is the purely physical osmotic activity of substances 

 in the cell-sap, it must not be forgotten that it also depends upon 

 the resistance offered by the protoplasm to filtration under pres- 

 sure : so that the maintenance of turgidity is after all a vital act. 

 The maintenance of turgidity appears, in fact, to depend upon a 

 certain state of molecular aggregation of the protoplasm lining 

 the cell-wall, in which it offers resistance to the escape of the 

 cell-sap ; whereas in the flaccid condition the state of molecular 

 aggregation of the protoplasm is such that it readily permits the 

 escape of the cell-sap under the elastic pressure of the cell-wall. 



Whilst the fundamental mechanism of the movement of mature 

 motile members and that of growing members is essentially the 

 same, there is this secondary difference between the two cases. 

 The change of position which is the result of the movement of 

 mature members, is reversible ; they can return to their former 

 position : the change of position, curvature for instance, of growing 

 members is reversible only so long as it has not been rendered 

 permanent by actual deposition of substance. Thus the changes of 

 position due to the nutation (p. 211) of growing members are only 

 temporary, for they are of brief duration ; but changes of position 

 due to some directive influence acting for a considerable time 

 become permanent, for instance, the light-positions (p. 223) assumed 

 by growing members. 



CHAPTER IV. 



SPECIAL PHYSIOLOGY OF REPRODCUTION. 



54. Introductory. It has been already stated (see p. 49) 

 that reproduction consists essentially in the throwing off by the 

 individual of a portion of its protoplasm which does not merely 

 grow but developes into a new organism ; and that two modes of 

 reproduction, vegetative multiplication and spore-reproduction 

 may be conveniently distinguished, though they are not absolutely 

 distinct. 



Reproduction has been considered so far mainly from the 

 morphological standpoint, and it now remains to discuss it from 

 the physiological point of view. The most important general 

 consideration is that reproduction is a function of embryonic, as 



