274 STUDIES FOR STUDENTS 



pointed out by Hilgard.^ The climatic point where mud-cracking 

 becomes broadly effective upon the clays of a flood plain is therefore 

 rather a critical one tending to separate the floral characteristics of 

 well-watered from subarid climates. 



Floral characteristics of semiarid flood plains. — One of the most 

 secure means at present commonly used to determine the climate of a 

 past age consists in the study of a fossil fauna and flora, the identifica- 

 tion of genera and species, and the inference that the optimum climatic 

 environment for such organisms has remained the same from the past 

 to the present time. As examples may be cited the conclusions in 

 regard to the warm polar climate of the Miocene based on the presence 

 of magnolias in Greenland and the same in Mississippian times as 

 determined by fossil corals in the rocks of Spitzbergen. Such strictly 

 paleontological sides of the problem are beyond the province of the 

 present article, but there may be profitably considered the relations 

 between climate and the kind of fossils to be expected and certain 

 general adaptive characteristics of plants or animals to arid or moist, 

 cold or warm conditions, especially when these are of a nature which 

 may be preserved. The present statements will be chiefly confined, 

 however, to the effects of semiarid climates upon vegetation. 



The vegetation of the flood plains of semiarid climates is more 

 largely arboreal than that of the inter-stream slopes.^ Many large 

 tracts of the flood plain away from the river banks are, however, 

 either sparsely covered with trees or given over to grass land. Even 

 the latter may find difficulty in existing where an unfavorable nature 

 of alluvial deposit is added to the unfavorable conditions of a hot or 

 dry growing season. ^ The occupancy of the soil by grass or forest 

 depends upon the underground water. For forests there must be an 

 adequate amount of moisture in the subsoil during the growing season, 

 though this water may have come from winter floods or rains. For 

 grassy plains the water in the subsoil is immaterial, the essential con- 



1 Soils, 1906, p. 112. 



2 A. F. W. Schimper, Plant Geography (Eng. trans. ), 1898, Map 3. 



3 Views of piedmont and terminal flood plains of the semiarid belts of the United 

 States may be found in various reports. For a study, with photographs, of the vege- 

 tation of the Rhone delta of the Mediterranean in a climate which approaches semi- 

 aridity see FlahauU et Combres, Sur la flore de la Camargue et des alluvions du Rhone, 

 Bulletin de la societe boianique de France, T. 41, 1894, pp. 37-58. 



