176 
As I have above pointed out, the summer rains commence 
about August 1. At this time the vegetation consists only of 
those perennials which are enabled to resist the drought. Besides 
the cacti, ies palo verde, and other low pe vegetation 
above referred to, there are only a few small plants, and almost 
no etna ones. Of the latter, only a very few scattered 
individuals of Auphordia and Astragalus reward search. A half 
woody Cassia and Encelia, a wholly composite, are perhaps the 
most abundant of smaller plants. At the end of two weeks, how- 
ever, a rich annual herbage has made its sudden appearance. Most 
of these have yellow flowers, and thus yellow becomes the domi- 
nant flower color. 7Zridulus, with its large poppy-like blossom, 
Martynia, with large fragrant flowers of the snap-dragon che 
and Cassia are most abundant. The flowers of Martynia 
offer another instance of pollination by means of small insects, 
while they are usually thought of as adapted especially to large 
ones, or even small birds. The — stigma is sensitive, 
and closes rapidly upon irritation. I have r iene noticed 
that these flowers are infested with aphides whic wl about 
on the stamens and stigmas, and thus by chance pee pollen 
on the sensitive stigmatic lips, which immediately close upon the 
adherent pollen. Similarly I have noted that the large tubular 
flowers of a Datura (D. meteloides) are constantly pollinated by 
honey bees, though night-flying moths may also be in part 
responsible, 
The season of summer rains is the spring time of the desert. 
The relative humidity increases materially, so that the heat be- 
comes rather oppressive, and desert conditions are no, more 
present. For the study of the desert vegetation proper there- 
fore, one must be at the laboratory during the prevalence of true 
desert conditions. August and September, while enriching the 
flora in point of numbers of plants, change the physiological 
conditions very materially. 
urning to the laboratory, I found that I was fortunate in 
having a unique opportunity for work. he a ant northern 
illumination, with the magnificent range of Hes ae Catalina 
mountains stretching out before the eye, plenty of working space 
