224 



The Plant World. 



Oregon, Indiana, Illinois, New York or Florida were decreased 

 gradually to one-third or one-fourth the average amount now 

 received, the total production of organic matter would be greatly 

 lessened and consequently the amount of hnmus would decrease ; 

 wind erosion would remove much of this from its place of forma- 

 tion and by this means alone the distribution of many species 

 would be totally altered. The soil moisture would ultimately 

 be so depleted that the surface layers would show as great a 

 proportion as the underlying layers, and carry an excess 

 during seasons of precipitation, a fact that would have the pro- 

 foundest influence upon the vegetation native to the region af- 

 fected, determining not only the habit of the root-system, the 

 form of the shoot, but also becoming a factor in distribution, 

 and giving a new form to the competitive' struggle among the 

 organisms in a locality. The change in precipitation would 

 result in the formation of long outwash, detrital slopes or ba- 

 jadas, piedmont to the mountains, giving new habitat conditions, 

 and a further differentiation would consist in the surface deposi- 

 tion of soil salts, giving alkaline and saline areas upon which 

 halophytic forms might flourish. The lessened relative humidity 

 would result in modification of foliar surfaces, make necessary 

 special structures in seeds and spores, and would be followed by 

 a more intense insolation by reason of the non-absorption of 

 some portions of the spectrum, and lastly, the course of the 

 temperature of the soil would change with the depletion of the 

 humus and the altered moisture relations. 



If desiccation ensued as a result of simple horizontal re- 

 duction of the precipitation, in a region w r ith an unbroken surface 

 lying at nearly the same height above the sea, the effect would 

 be sweeping, monotonous, and with almost total absence of 

 selective effect that would mean extermination, or change in a 

 flora en bloc. The majority of interpretations of the paleanto- 

 logical record assume such results. It is to be seen, however, 

 that desiccation in a region with diversified topography and great 

 differences in level would result in great differentiation, and if 

 this reduction took the form of limiting the rainfall to one or 

 two brief seasons, or limited periods, a maximum of effect might 

 be expected. 



