144 Diffusion and Osmotio Peessukb 



iii. summary of the chapter 

 As far as investigation has gone, it has been found that 

 growth is accelerated in weak solutions and retarded in con- 

 centrated ones. The term "growth" here includes, not only 

 enlargement, but also the process of cell division. Also, in 

 some cases at least, the direction of new walls is profoundly 

 influenced by the concentration of the surrounding medium. 

 In general, all vital processes are retarded in concentrated 

 solutions. Reproduction, being a peculiar form of cell divis- 

 ion, appears in some cases to be entirely dependent upon the 

 osmotic pressure of the surrounding medium. Irritability is 

 also greatly influenced by external pressure. Not only is 

 this function retarded in concentrated solutions, but in some 

 forms the direction of response to a given stimulus may be 

 reversed by a sudden change in the osmotic surroundings. 

 The comparative concentration of the external and internal 

 solutions acts, in many cases, as a stimulus upon the organ- 

 ism, giving rise to the phenomena of osmotaxis. 



All the effects of high concentration of the surrounding 

 liquid seem to be due to extraction of water from the living 

 cells. They may be due either to a drying-out process or 

 to decrease in turgidity. That they are sometimes due to 

 the former is proved by curious analogies between the vari- 

 ous processes which extract water from the protoplasm. 

 Whether or not this extraction of water from the protoplasm 

 itself is the direct cause of the responses to concentrated 

 solutions, is not yet known. The effect may be a chemical 

 one, due to the increased concentration of the contained 

 solutions. 



