378 ENVIRONMENTAL CONDITIONS. 



of table 22 shows that the indices themselves are markedly different 

 in the three cases, however. The values indicated for the hnes of 

 plates 70 and 72 are thousands, while those for the lines of plate 71 are 

 hundreds, and the table shows that the indices obtained from the 

 exponential summations are always the smallest of the three. Those 

 obtained from the remainder summations are roughly 10 times as 

 great as those from the exponential ones, and those from the physio- 

 logical summations are about twice as great as those from the remainder 

 summations, or 20 times as great as those from the exponential ones. 

 These statements are only roughly approximate, however, so that the 

 values indicated on the three charts are: from 1 to 13 thousands, from 

 1 to 15 hundreds, and from 1 to 24 thousands, respectively. It is thus 

 seen that the actual values given for any given station are quite differ- 

 ent on these three charts, but, as has been said, the general zonation is 

 nearly the same for all. 



Peninsular Florida, a small area about Cape Hatteras, and the very 

 humid Northwest show the highest values. (See fig. 18, derived from 

 plate 72.) The Pacific coast region south of the humid Northwest has 

 relatively low values, which are similar to those of the plains just east 

 of the Rocky Mountains. The Great Basin region and the arid deserts 

 of Arizona and southern Cahfornia belong in the area of lowest index- 

 values. From the plains belt eastward the values increase to about 

 the one-hundredth meridian of west longitude. East of this meridian 

 the hnes of the charts roughly parallel the gulf and Atlantic coasts. ^ 



It is at once clear that these moisture-temperature charts combine 

 the features of moisture charts with those of temperature charts. The 

 index values here brought forward are predominantly controlled by 

 moisture conditions in the arid regions and similarly controlled by 

 temperature conditions in the cold regions, while for regions of inter- 

 mediate moisture and temperature conditions, neither the one nor the 

 other group is dominant in this control. 



As Livingston has pointed out, it is not to\be expected that this 

 climatic zonation will he found to correspond generally with the dis- 

 tributional zonation of plants; these charts indicate, for example, that 

 Portland, Maine; Milwaukee, Wisconsin; Dodge, Kansas; and Ama- 

 rillo, Texas, all belong in the same chmatic zone. This system of indices 

 was planned, however, to represent, not plant distribution, but the 

 chmatic possibility of plant growth, in general. The most valuable use 

 of these charts will doubtless come in studies of annual plant produc- 

 tion, without particular regard to the forms of plants involved. They 

 will probably be most useful in connection with agricultural and 

 forestal studies. In this connection Livingston has remarked: 



"If it were possible to improve the temperature conditions (length and temperature 

 efficiency of the season of plant growth) for Portland until they were as good as those for 

 Tampa, Florida, then the potential annual plant product per acre (aside from soil influences) 

 for the northern station should about equal that for the southern. But temperature con- 



