390 CORRELATION OF DISTRIBUTIONAL FEATURES. 



In the United States there are many plants which appear to have 

 three sets of controlling factors, one limiting them toward the north, 

 one toward the south, and one toward the west. 



The investigation of distributional limits is still further complicated 

 by the fact that the potency of a given condition in limiting the geo- 

 graphic range of a plant may vary with the intensity of some other 

 condition or conditions. This is very well shown in the case of the 

 westward extension of Quercus macrocarpa (see plate 18), which reaches 

 eastern Montana in the north and extends only to central Oklahoma 

 in the south. The fact that this tree reaches its western limit in 

 flood-plains points to the influence of water-relations in limiting its 

 western extension, as is doubtless true of all trees of the Deciduous 

 Forest region. All climatic lines which have to do with the water 

 relations, however, have a nearly north-and-south direction in this 

 part of the United States. It is obvious that a given set of moisture 

 conditions may permit the growth of Quercus macrocarpa in South 

 Dakota, while the same moisture conditions, under much more exacting 

 conditions of temperature, and consequently of evaporation, would 

 inhibit the occurrence of the tree in the Panhandle region of Texas. 

 Similar considerations are doubtless involved in the western limit of 

 the Grassland area. 



Another case of this extremely common state of affairs is to be found 

 in connection with the northward distributional limit of Opuntia 

 missouriensis. This plant, like many of its congeners, appears to have 

 its northward distribution determined by some phase of the winter- 

 temperature conditions. The cacti appear to be able to withstand 

 low temperatures much better when their water-content is low than 

 when it is high. In the dry late summer of the Grassland region 

 Opuntia missouriensis becomes desiccated to such an extent that it is 

 able to withstand the winter temperatures up to a latitude of 53°. In 

 the regions to the east and west of its northernmost area the winter 

 conditions are scarcely more severe, but the soil-moisture conditions 

 of the late summer are such as would bring the cactus to the winter 

 season in a state of turgidity that would prove fatal. This is a case, 

 in brief, in which the annual march of soil-moisture conditions appears 

 to cooperate with the winter temperature conditions in determining 

 the limitation of a species. 



There are very many cases in which it is possible to demonstrate 

 that a particular climatic condition at a particular intensity is responsible 

 for the position of a distributional limit. But in no case does the opera- 

 tion of this factor fail to be influenced by the intensities of other 

 factors. The most that we can do is to analyze the environmental 

 complex, to discover which of the various environmental conditions is 

 of the greatest relative importance in determining a given distribution. 



The coincidence of a charted distributional limit and an isoclimatic 

 line can not be taken as an absolute and logically complete proof that 



