116 
J. C. Th. Uphof. 
The relative humidity differs greatly, not only with the time 
of year, but also at different parts of the day. On very hot days 
at about 2 p.m. it ranges from 25 to 35, between 5 and 6 a.m. from 
80 to 90, at 10 p.m. from 35 to 50. The number of cloudless days 
during the year is considerable, amounting to over 320. 
Meteorological conditions of semi-arid regions in the Old 
World are similar to those in Southern Arizona. As the 
xerophytic plants which the author has studied are also to be found 
in dry regions of Asia and Africa, it is useful to give data relating 
to these parts of the world as far as could be obtained. Most of 
the data were obtained from the Meteorologische Zeitschrift. 
Temperature, Relative Humidity and Rainfall of various Deserts. 
JC 
c 
o 
£ 
North Africa and 
South West Asia. 
Algerian Sahara. 
West Central 
China. 
South African 
Desert. 
Max. Temp. 
Centigr. 
Relative 
Humidity. 
Rainfall 
mm. 
Max. Temp. 
Centigr. 
Relative 
Humidity, 
Rainfall 
mm. 
Max. Temp. 
Centigr. 
Relative 
Humidity. 
Rainfall 
mm. 
Max. Temp. 
Centigr. 
Relative 
Humidity. 
Rainfall 
mm. 
Jan. 
24-8 
81-5 
44-8 
20-7 
48 
19 
10*5 
82 
18 
21-7 
90 
M 
Feb. 
29*8 
860 
190 
23-8 
31 
7 
14-0 
57 
0 
23-9 
92 
1*6 
Mar. 
20-2 
83*0 
8-1 
30-7 
26 
15 
24-5 
65 
23 
29-4 
92 
3 2 
April 
20-1 
83*0 
7-1 
34-3 
21 
8 
33-7 
40 
13 
34-4 
84 
5-0 
May 
20-2 
92-0 
0 
39*0 
17 
17 
35-7 
25 
13 
33-3 
90 
14-5 
June 
22-9 
91*0 
0 
45* 1 
14 
1 
38-2 
19 
0 
28-3 
86 
7-9 
July 
22-3 
93-0 
0 
47-4 
12 
0 
405 
29 
5 
25-6 
88 
5-2 
Aug. 
26*7 
94-0 
0 
46-6 
15 
2 
35-5 
30 
6 
26-1 
87 
68 
Sept. 
28-7 
92-0 
34-7 
42*6 
19 
4 
36T 
26 
0 
39-4 
79 
5-6 
Oct. 
25-9 
92*0 
55-9 
36-7 
31 
3 
21-9 
35 
0 
32-8 
76 
2-8 
Nov. 
25*7 
890 
38 1 
27-1 
37 
10 
22-0 
39 
1 
37-2 
80 
2-2 
Dec. 
250 
87-0 
17-3 
21*1 
45 
28 
177 
67 
10 
38*3 
88 
1*1 
Whole 
Year 
225 
114 
89 
57 
The reflection of light from the underside of a leaf of 5. pilifeva 
or S. lepidophylla , as seen under the low power of the microscope, 
is confined to the ventral leaves. Chlorophyll and living 
protoplasm are absent, the cells are colorless, with the exception 
of these cell-groups exposed to the extreme action of the sun, which 
have their walls impregnated with a red-brown pigment. Only 
small sections which are covered by a part of the succeeding 
leaf remain green and are alive. In their natural environment the 
ventral leaves become mainly agents of reflecting light, instead of 
producing organic compounds under influence of the light. This 
production is taken over by the entirely green dorsal leaves, which 
explains their being in comparison to the ventral leaves much 
larger than those of the hygrophytic species. Fig. IX, 1, 2 shows a 
