60 



ihH 



perhaps quite as important as water-supply or tempera- 

 ture. It will be necessary to distinguish between 

 habitats on the basis of the good or bad aeration of 

 the soil and between species on the basis of their vary- 

 ing susceptibility to these conditions. 



For the species investigated so far there is good 

 correspondence between the sensitiveness to aeration and 

 the ecological habit. Coleus and heliotrope, which are 

 very sensitive are known to prefer open and well- 

 drained soils in which aeration is presumably good. On 

 the other hand the willow, which is not sensitive, is 

 a swamp plant capable of growing where aeration is certain- 

 ly very poor. The same is true of the eucalyptus though 

 this plant has a much wider distribution than the willow, 

 being capable of growth on well-drained soils as well 

 as in poorly drained ones. Probably this difference 

 is due to the relations of the two species to water. The 

 willow is insensitive to aeration but very sensitive to 

 water-supply, being most at home only in the wetter soils. 

 The eucalyptus is comparatively insensitive to water- 

 supply as well as to aeration. The oleander, which is 

 intermediate in sensitiveness to aeration is also in- 

 termediate in distribution, preferring neither the ex- 

 treme openess suitable to Coleus and heliotrope nor the 

 extreme wetness endured by the willow and eucalyptus. 

 The desert plants Opunfciaand Prosopis show a similar 



correspondence between oxygen requirement and distribution. 



