270 LIVINGSTON— CLIMATIC AREAS [April is, 



both the tendency of the plant to gain heat and its tendency to lose 

 heat, we find no such simple climatic factor to use in studying the 

 conditions which tend to add water to the plant or to remove it. 

 As has previously been mentioned, the ordinary plant derives most 

 of its moisture supply from the soil and loses water to the air. The 

 possible rate of moisture supply to growing plants is thus determined, 

 by the resistance of the soil to the movement of moisture into plant 

 roots. While the physical properties of the soil play an important 

 part in this connection and while these vary from place to place, the 

 amount of water present in the soil is also of primary importance. 

 This depends, for any particular soil and in the majority of cases, 

 upon precipitation, and the measurement of this climatic factor 

 furnishes us, as is commonly recognized, with a criterion of consid- 

 erable value in the comparison of climatic areas, ^^'hile the distri- 

 bution of rainfall throughout the period of the plant's activities is 

 fully as important as its amount, I shall give attention in this paper 

 only to the latter. 



It has already been emphasized that the evaporating power of the 

 air is the main climatic feature in the control of water loss from 

 plants, as from other moist objects. If we add to this the water- 

 extracting or desiccating power of the sunshine we have an exceed- 

 ingly satisfactory measure of the water requirements of plants, for 

 most of the water absorbed by ordinary plants is lost by transpira- 

 tion. Here also I shall consider only the question of the mean 

 evaporating power of the air throughout the period of the frostless 

 season. 



If we assume for the moment that soils are all alike in their 

 physical properties, and if the moisture supply of plants be propor- 

 tional to precipitation while the water loss is proportional to the 

 evaporating power of the air, some relation obtaining between these 

 two factors should be a direct measure of the vegetational water 

 relation. Unfortunately for our study, the assumptions above made, 

 especially the one regarding the physical properties of soils, are very 

 far from true ; yet certain physical types of soil are found in every 

 one of the climatic areas which we are apt to encounter, and for 

 any such t}pe the relation just referred to should be of great value. 



