GROWTH 15 



due during the first few hours principally 

 to imbibition, after that principally to 

 osmosis. We should therefore expect 

 that the dead seed would soon stop 

 growing while the living one would 

 continue. 



Osmosis does not ordinarily develop 

 so much pressure as imbibition but it is 

 supposed that the pressure it produces 

 in the living cell may reach three hun- 

 dred pounds per square inch or even 

 more: this is as much as is commonly 

 found in steam boilers. It is sufficient 

 to drive ferns up through macadamized 

 roads and concrete sidewalks and to en- 

 able toadstools to lift heavy flagstones. 



Let us now consider whether there is 

 anything in growth which can be used 

 as a criterion of life. We have tried first 

 of all to discover what is essential to 

 growth. Such things as cell division, 

 change of form, differentiation, and the 

 assimilation of food may be taken away, 

 and yet growth may go on for a long 

 time. One process cannot be dispensed 



