150 ENVIRONMENTAL CONDITIONS. 



influencing plants. This is not always the case, however, for the air 

 temperature of the climatologist and meteorologist is the temperature 

 condition of the aerial environment of organisms, and the evaporating 

 power of the air is a factor that directly affects the rate of water-loss 

 from the aerial parts of plants and animals. Both of these immediate, 

 and thus truly environmental, conditions we have been able to con- 

 sider to some extent. On the other hand, such climatic factors as 

 rainfall, humidity, vapor-tension of water, wind velocity, and duration 

 of sunshine are all recognized chmatic factors, concerning the distribu- 

 tion of the various intensities of which many data have been accumu- 

 lated, but which have no direct influence upon plant activities. These 

 climatic factors are very important, however, and often exert a con- 

 trolling influence upon the more directly effective environmental 

 conditions. Thus, the partial pressure of water-vapor in the air and 

 the rate of air-movement influence the evaporating power of the sur- 

 roundings. Rainfall greatly influences soil-moisture, and hence the 

 ability of the soil to supply water to root surfaces, but it does not 

 determine this environmental condition, for other factors, such as the 

 physical nature of the soil, its exposure, subterranean water-flow, 

 etc., must be taken into consideration in this connection. It therefore 

 became necessary not to restrict our studies to immediately effective 

 conditions, but to consider in most cases the more remote climatic 

 factors which meteorology and climatology have placed at our disposal. 

 The subterranean environment of plants has not, as yet, been 

 studied in any way at all adequate to the present purpose, and our 

 knowledge of the relation of this to plant distribution is still in the 

 first stages of the purely observational phase. It has therefore been 

 impossible for us to devote serious attention to this exceedingly im- 

 portant category of environmental conditions. Nevertheless, on 

 account of a general similarity of the prevailing soils of most of the 

 broad vegetational areas of the United States, our studies of the rela- 

 tion between the available measurements of the aerial conditions in 

 connection with vegetational distribution are not as unsatisfactory as 

 , they might otherwise be. In the majority of the great vegetational 

 areas with which we have to deal, the prevailing soil is a clay or a 

 clay loam, with usually a rather deep-lying subterranean water- 

 table, and with a more or less pronounced admixture of organic 

 matter, and the prevailing vegetational types are, in the majority of 

 cases, found upon this character of soil. Exceptions to this generaliza- 

 tion are swamps and marshes on the one hand and sandy regions on 

 the other. The general vegetational type of broad marshlands appeals 

 to be about the same for a great range of aerial conditions; thus, these 

 bear the same physiological types of plants under the climatic con- 

 ditions of Baja California as under the very different ones of the 

 southeastern coast. Also, the physiological characters of many plants 



