408 BOTANICAL GAZETTE [JUNE 
heaviest rainfall occurs in summer. These facts encourage the view 
that the peculiar structure of the plants of the Egyptian-Arabian 
deserts, as regards the points under consideration, may likewise be 
due wholly or in part to analogous causes. 
The climate of these foreign deserts is quite different from that 
of the desert about Tucson 
| | LT | in the distribution, as well 
j / % as the amount, of the 
\ annual rainfall; and conse- 
/ \ quently in the relation of 
rf \ the rainfall to the period 
/ i of the highest temperature. 
i \ The most rain occurs in 
f the deserts of northwest- 
4 ern Africa and southwestern 
4 Asia in winter, but the 
\ 
maximum temperature is 
in summer. 
The period of the 
greatest vegetative activity 
in these deserts is in win- 
ter; in summer the plants 
Fic. 10.—Rainfall and temperature at 4!€ dormant. Inthe Egyp- 
Cairo, Egypt, in 1887; broken line is temper- tian-Arabian deserts, there- 
ature (1¢™=2°C.); solid line is rainfall ‘ th 
(rem = mm), fore, the renewal of growt 
N 
occurs at the time of the 
year when the rainfall is also greatest, but not at the season of 
the highest temperatures. The curve of growth of the plants of 
this region probably coincides very accurately with the curve of the 
yearly rainfall. I have no means at hand of telling whether, on the 
other hand, if such plants were irrigated the period of their greatest 
growth would fall insummer. Such is apparently the case with some 
of the cultivated forms’ in deserts farther west. 
Fig. 10 represents the relation of the yearly rainfall to the tem- 
perature at Cairo, for the year 1887. 
5 SWINGLE, W. T., The date palm. U.S. — Agric. 1904. 
DESERT BOTANICAL LABORATORY. 
