12 THE NATURE OF LIFE 



which results in continually giving off 

 carbon dioxide to the air. In this way 

 nearly half the dry material may disap- 

 pear during growth. 



It is true that growth must eventually 

 cease under these circumstances but the 

 fact that it can go on for so long although 

 the plant takes in no food shows that 

 increase in dry weight is not necessary 

 for growth. 



Since we find that growth may occur 

 without increase in dry weight, change 

 of form or color, cell division, or differ- 

 entiation, we may ask. What is really 

 essential to growth.'^ The answer seems 

 to be. An increase in size due to the ab- 

 sorption of water. Let us now look into 

 this more closely. 



It is a very striking fact that when 

 dry seeds are planted in moist soil the 

 dead seeds appear to grow in the same 

 way as the live ones during the first few 

 hours. We find, however, that a dead 

 seed soon stops growing while the living 

 one continues. This suggests that the 



