346 THE FUTURE OF ARID LANDS 
TABLE 1 
Effect on Vegetative Growth of Supplying Soil Moisture 
Supplied at weekly intervals throughout the dry summer period to twenty perennial 
species in the field at Davis, California, 1947-1950 (32). 
Species Ceasing Vegetative Growth 
Species Continuing Vegetative Growth Retaining green Retaining no 
tissue green tissue 
Agropyron desertorum Melica californica Poa bulbosa 
Bromus carinatus Poa nevadensis Poa scabrella 
Bromus catharticus Stipa cernua Poa secunda 
Bromus stamineus Stipa pulchra 
Dactylis glomerata 
Ehrharta calycina 
Elymus glaucus 
Festuca arundinacea 
Lolium perenne 
Oryzopsis miliacea 
Phalaris tuberosa var. stenoptera 
Poa ampla 
Poa compressa 
to the permanent wilting percentage to a depth of 6 feet. Undis- 
turbed plants showed considerable foliage development by Sep- 
tember 12, whereas those with roots severed at the 2- and 4-foot 
depths did not resume growth until after rain had fallen late in 
October. 
In Poa scabrella, however, the initiation of summer dormancy 
is associated with long photoperiods and high temperatures. Cool- 
ness and availability of water break this dormancy. 
It is seen from these studies that different environmental 
factors, singly or in combination, are associated with the initiation 
and the breaking of summer dormancy: moisture availability, 
temperature, and day length. 
The drought resistance of a plant, then, is a reflection of the 
interaction of a number of physiological and morphological re- 
sponses to the many environmental factors opereting during the 
life history of the plant. 
One aspect, to my knowledge, has not yet been sufhiciently 
investigated. I refer to the effect of soil type on the survival capa- 
bilities of different species in arid areas. For example, our field 
