BETTER ADAPTATION OF PLANTS 345 
Similarly, attempts to explain drought resistance under con- 
trolled environments on a purely physiological basis have not to 
date been entirely satisfactory, although certain facets have been 
explored. Aamodt (1) and others are of the opinion that drought 
resistance must be determined by direct testing of the plants 
under controlled conditions. The difficulty has been that any one 
investigator has limited his study to one or two factors, whereas 
in the field many factors may be operative at the same time. For 
instance, high temperature is one facet of drought, but the ability 
of a plant to withstand the effects of high temperature is not 
necessarily correlated with the ability of a plant to survive in 
arid conditions. Still, high temperature is one factor and studies 
by Julander (29) and others have indicated that in some species 
resistance to high temperatures may be taken as an index of 
drought resistance. 
In regions where plants are subjected to lengthy periods of 
summer drought and heat, perennial plants must in some manner 
withstand the severity of the summer period. During such a period 
plant growth becomes very limited or ceases entirely. Laude (32), 
using 20 perennial grasses, conducted a critical study of the nature 
of this condition which he termed ‘‘summer dormancy.” With its 
Mediterranean type climate a dry summer period of 5 to 6 months 
prevails at Davis, California. Normally, all 20 species are summer 
dormant under natural rainfall conditions. Thirteen of the 20 
species continued vegetative growth throughout the summer when 
watered at weekly intervals; four species ceased vegetative growth 
even though supplied with water, although they did retain green 
tissue; and three species did not even retain green tissue (Table 1). 
Another test was devised to allow a period of summer dormancy 
and then supply water to the seven species that failed to continue 
growing in the first test. Four of the seven commenced growth 
after a short dormancy period, but two species of Stipa did not 
respond to water until September, and Poa scabrella did not break 
dormancy until cool weather prevailed with the first fall rain. 
In still another test, Laude severed the lateral roots 18 inches 
from the center of the rows, and the vertical roots at 2- or 4-foot 
depths by cuts under the blocks. This was done in late July. Soil 
moisture samples taken August 3 indicated that the soil was dry 
