230 



The Plant World. 



the moist seasons of an arid period, so the surviving zerophytes 

 in a moist period would find refuge in restricted habitats on 

 talus slopes, rocks and sand, in which the soil-structure relations 

 would be best suited to their specialized structure and might 

 display their seasonal activity during a period of the year in 

 which the precipitation was least. 



GENERAL CONCLUSIONS. 



In a brief summarization of the main features of the subject, 

 it is to be recalled that free water was a very important agent in 

 origination of self -generating matter, and that the development 

 of the vegetal organism up through the gametophytic stage was 

 accomplished in its presence. 



It was in the stress of aridity encountered in extended and 

 elevated land areas, however, that the development of the 

 sporophyte, with its highly differentiated vascular system, com- 

 plicated physiological organization and seed-forming habit, oc- 

 curred. 



Certain geological formations laid down at a time when 

 the prevailing types of vegetation were characterized by separate 

 gametophytes are devoid of fossils. Fossil xerophytes are un- 

 known, although fair evidence of aridity in the Cretaceous and 

 earlier is at hand. Certain groups of plants, however, including 

 the cycads, Bennetitales, and conifers show structures which 

 would be suitable for existence under arid conditions, although 

 not all of the forms so equipped inhabit arid areas. 



Rock formations indicate the prevalence of arid conditions 

 over extensive areas as far back as the records may be interpreted. 

 The desiccated regions coincide only in part with deserts of the 

 present time. Desert conditions appear to have prevailed in 

 central Asia, north and south Africa, western and eastern South 

 America, parts of Australia, and southwestern America 

 since Cretaceous times. In the Lop basin of Asia, the central 

 valley of Arabia, portions of the Sahara, the Kalahari in southern 

 Africa, the Lake Eyre basin in southern Australia, the Salton 

 basin in California, and the elevated basins of Nevada and Utah, 

 the characteristic vegetation is composed largely of spinose 

 and switch-like forms, in which the chief development has been 

 toward restriction and induration of surfaces; a result attribut- 

 able to the degree of aridity, the seasonal distribution of the rain- 

 fall and also to the intervention of great climatic oscillations. 



