194 NATURAL HISTOEY BULLETIN 



ance with the well-known tendency of moving gases to follow 

 somewhat irregular surfaces,^" whereas when the northern slope 

 is more abrupt the prevailing southwesterly winds often pass over 

 without following it. 



In the latter case, however, there is sometimes a division of the 

 current in which case a part of the current sweeps back up the lee- 

 ward slope. This occurs in rough territory where ridge after 

 ridge intercepts the air-currents and causes disturbances in their 

 lower strata, and was observed only where a ridge in front caused 

 this deflection. The writer has frequently tested this by releasing 

 bits of paper, the pappus of composites and other light objects 

 from the summit of such ridges, and invariably when strong 

 winds were blowing most of these objects were swept upward 

 again, often to the feet of the experimenter. In such cases the 

 leeward slope is almost as much exposed to the dry winds as the 

 windward side, and it is then wholly or largely prairie. 



In all cases where the topography of the prairie is rough the 

 rapid run-off of the rains precipitated upon the steep slopes as- 

 sists in more rapidly making the surfaces xerophytic. That this 

 is not the prime cause, however, is evident from the fact that 

 steep slopes (especially those facing the north or northeast) are 

 densely covered with a mesophytic forest-growth where sheltered 

 from the winds. Excellent illustrations are furnished by practi- 

 cally every timbered hollow or valley in the Missouri river terri- 

 tory, for all are similarly situated. The fact that the groves are 

 practically restricted in this and similar territory to the rough- 

 est areas is also worthy of consideration.^^ 



. The striking distribution of the groves on rougher lands in the 

 western part of Iowa and elsewhere suggested a series of obser- 

 vations on the relative rate of evaporation from the sheltered and 

 protected slopes. The bluffs and ridges bordering the Missouri 



" Plate V, figure 2 illustrates such prairie areas on gradual northern 

 slopes. 



" The clumps and narrow belts of trees along the larger streams on the 

 broad alluvial plain of the Missouri river form an exception to the rule. 

 The diffusion of moisture from the stream in both soil and air evidently 

 makes this possible as the plants in these belts are practically all meso- 

 phytic. 



