ro BOTANICAL GAZETTE [SEPTEMBER 
that of the subjacent valley. The seasons of greatest and 
least rainfall were synchronous. 
But for the purposes of the present inquiry the extreme of 
moisture and drought to which plant life is exposed is of as 
great importance as the average amount of rainfall; and the dis- 
tribution in time of this amount is a factor of equal value. 
Again it will be found that in these respects the desert vegetation 
is at a disadvantage. Throughout the deserts total yearly rain- 
falls of 1.5 to 4™ (38-101 ™) are the rule, and at some stations 
a whole year may pass with absolutely no measurable precipi-— 
tation. But this small amount is so evenly distributed through 
the year as to produce much less effect, certainly for annual 
plants, than if it were concentrated in a few months. The table 
below shows the normal distribution of rainfall throughout the 
year at two desert stations, compared with that at two stations 
on the coast. 
NORMAL MONTHLY PRECIPITATION IN INCHES. 
a - my = | o Poy ; ic 
STATIONS $igicisSlB| Fl ele| 2) els 1b) 873 
>lsle(Si<c(/S/S5/4S (al alo;4i/ale 
Keeler 8 \ 0.2 0.4|0.2 0.3|0 4|0.2 0.1] 0.2/0.3] 0.3/0-4)9-4] 3-3 
Yuma. 16 }0.4]0.6\0.2/0.1] T | T Jo.1]0.4]0.1]0.3|0-3]9-6) 3-2 
San Diego 42 |1.6|2.1| 1.0] 1.0] 0.3|.0.1]0.14] 0.1] 0.1] 0.3] 1-0] 2-2 9.8 
ucainaiebiiaeectn As on ce (hs saa ba ed pe ee T |o.r| T |0.7| 1.6) 3-7|18-5 
It here appears that the scanty rainfall of the desert stations 
is quite evenly distributed through the whole year, while at 
San Diego 9.1 inches out of the total 9.8 inches, and at Los 
Angeles 17 inches out of 18.1 inches fell during the growing 
season between November and May. That is, over 95 per cent. 
of the total rainfall at these places comes at a time when, owing 
to a lower temperature and a more declined sun, it is not subject 
to rapid evaporation, and avails most for the growth of vegeta- 
tion. On the other hand almost all the desert rainfall may be 
regarded as ineffective by reason of its equal distribution. 
It must be remembered, however, that all our recorded obser- 
vations were made at stations in the open deserts, whereas. 
had they been made in the cafions of the circumscribing moun- 
tains, somewhat more favorable results would have been obtained. 
= paeaaR 
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