10 



PLANT PHYSIOLOGY AND ECOLOGY 



diminished by the presence of excessive amounts of salts in the soil, 

 and also by unusually low temperatures. Hence in measuring the 

 water content of habitats the best practice is to determine both 

 holard and chresard, bearing in mind that it is the latter alone that 

 results in adjustment and adaptation. 



13. Soil samples. In obtaining samples of soil for finding the 

 water content, the usual practice is to remove the air-dried sur- 



FiG. L Geotomes and soil can, showing at the left the plunger for remov- 

 ing the soil core- 

 face, noting its depth, and then to sink the soil-cutter or geotome, 

 with a slow, boring movement, in order to avoid packing the soil. 

 This tendency is further reduced by cvitting a long core a decimeter 

 at a time. As soon as the sample is dug, the plunger is used to 

 press the core from the geotome directly into an air-tight soil can. 

 The lid is screwed on as quickly as possible, and the number recorded 

 immediatelv with such notes as are desirable. The cans should 



