208 DRY-FAEMING 



forms about one fifth of the atmosphere and is the 

 active principle in combustion and in the changes in 

 the animal body occasioned by respiration. Oxygen 

 should be present in the soil air in approximately the 

 proportion in which it is found in the atmosphere. 

 Germination is hindered by a larger or smaller pro- 

 portion than is found in the atmosphere. The soil 

 must be in such a condition that the air can easily 

 enter or leave the upper soil layer ; that is, the soil 

 must be somewhat loose. In order that the seeds 

 may ha^'e access to the necessary oxygen, then, sow- 

 ing should not l^e done in wet or packed soils, nor 

 should the sowing imj^lements be such as to press the 

 soil too closely around the seeds. Well-fallowed soil 

 is in an ideal condition for admitting oxygen. 



If the temperature is right, germination begins by 

 the forcible absorption of water by the seed from the 

 surrounding soil. The force of this absorption is 

 very great, ranging from four hundred to five hun- 

 dred pounds per square inch, and continues until the 

 seed is completely saturated. The great vigor with 

 which water is thus absorbed from the soil explains 

 how seeds are able to secure the necessary water 

 from the thin water film surrounding the soil grains. 

 The following table, based upon numerous investiga- 

 tions conducted in Germany and in Utah, shows the 

 maximum percentages of water contained bj' seeds 

 when the absorption is complete. These quantities 

 are reached only when water is easily accessible : — 



