"7. — Other assumed causes, such as the bison, sea, etc., are 

 of remote interest and not to be taken into account in any at- 

 tempt at any explanation of the prairie as a whole. ' ' 



Since the publication of that paper much additional evidence 

 has been secured by the writer and his assistants, Messrs. Gid- 

 dings and Boot, — a large part of it in the region under discus- 

 sion. This e^adence has only reinforced that previously secured, 

 and has led to no important modification of the earlier conclu- 

 sions, though an explanation of one of them may be desirable. 



A literal interpretation of the fii'st of these conclusions might 

 give the impression that evaporation directly affected the exposed 

 plants on prairies by causing a loss of water, and perhaps that 

 this loss was directly proportional to the rate of evaporation, — 

 though this was not intended. Repeated measurements of the 

 amount of transpiration from prairie plants have shown that on 

 the hot prairies when evaporation is greatest transpiration is 

 least.' This, together with the evident depression of acti^aty 

 shown in wilting, etc., suggest that the physical factors which 

 make for greater evaporation check the general activity of the 

 plant and cause a diminution in transpiration, the latter being 

 evidently merely a result and index of that activity. The wind 

 exerts an influence not only by aiding evaporation, but also by its 

 physical impact against the plant. Violent winds jar a plant, 

 check its activity, and the result is soon shown in diminished 

 transpiration. It is interesting to note that some of the prairie 

 plants will stand much more of this jarring than structurally less 

 protected mesophytic plants. Thus the writer has found that in 

 indoor experiments vrith the leaves of CUvia, exposed to currents 

 of air from electric fans, transpiration increased quite regularly 

 with evaporation with increasing wind velocity up to about six 

 miles per hour, and when the wind became more violent transpir- 

 ation dropped, though evaporation still increased; while with 

 some of the prairie plants the dropping of the transpiration curve 

 did not begin until a velocity of twelve miles per hour had been 

 passed. It is therefore evident that the physical factors acting 



5 While much -work on transpiration of prairie and other plants has been done by 

 the writer and his assistants, only a fragment has been published in Giddings' paper 

 on "Transpiration of Silphium laciniatum L., and another was presented by Boot 

 before the Atlanta meeting of the American Association for the Vdvancement of 

 Science," and in part published in the Proceedings of the Iowa Academy of Sciences, 

 vol. XXI, pp. 12.5-126, 1914. 



