Aridity and Evolution. 



223 



for gametophytic reproduction were very favorable, and a lux- 

 uriant development of ferns and related forms became possible. 

 aridity, the most important factor affecting the develop- 

 ment OF THE PLANT. 

 The seed-plant with its pollen tube and vascular system 

 may be considered as the major vegetational response to limi- 

 tations of the supply of moisture, and desiccation has been and 

 continues to be the most important condition affecting the evo- 

 lutionary development of plants. Temperatures alone have 

 been unduly drawn upon in the interpretation of distributional 

 features of ancient and existing floras, a fact made more plainly 

 apparent by recent observations at the Desert Laboratory, in 

 which it has been found that several species range over a vertical 

 mile. Such species endure cold of -35° C, and have a growing 

 season of less than a hundred days in the more boreal or alpine 

 portion of their ranges, while in the southern or lower localities 

 inhabited by them, temperatures of 48° C. may be encountered, 

 the growing season extending over 300 days; the thermometer 

 oing below the freezing point not more than 12 hours during the 

 entire year. 



It is no surprise, therefore, that it is learned that there is 

 no single feature in the structure and functionation of plants 

 that with perfect assurance may be connected with the influence 

 of temperature alone, although alpine and polar floras bear a 

 distinct aspect by reason of a combination of conditions of 

 moisture, insolation, duration of the seasons and course of the 

 humidity. 



While temperature is not in itself a direct factor in shaping 

 the general trend of evolutionary development in plants, yet it 

 is indirectly concerned by the influence exerted upon precipita- 

 tion, and the relation of the amount of rainfall to the possible 

 evaporation. The great periodic changes in climate, produced 

 by whatever cause, may be taken to have affected vegetation 

 chiefly through the desiccation effects, which not only deter- 

 mined the ranges and habitats of forms but also played a pre- 

 dominant part in evolution. 



GENERAL EFFECTS OF DESICCATION. 



The consequences of a decrease of the supply of moisture in 

 any region are very complex. If, for instance, the rainfall in 



