Physiological Anatomy of Xerophytic Selaginellas. lib 
Both species prefer exposed, rocky situations, especially 
with a northern aspect. In the neighbouring flat rockless deserts 
they are never met with. The plants are subject to extreme 
drought and intense heat; for a large part of the year under such 
circumstances they are to be found curled up into a big loose 
cluster-ball; the underside of the plant, which is alone exposed, 
differs in color in proportion to the amount of its exposure to the 
full severity of the sun. When not so exposed, owing to protection 
by the shadow of big rocks, this underside is almost white; the 
stronger the effect of the rays on the plants, the more the 
underside of the leaves will contain a brown to red-brown color. 
When illuminated by the sun’s rays, the clusters appear to be 
more or less glossy on account of the light being reflected by the 
white or brownish cells. The clusters are sometimes detected with 
difficulty at small distances on account of the similar color of 
surrounding rocks and are thus easily overlooked. 
The air temperature, the rainfall and relative humidity to which 
thespeciesof thisgroupareespecially subjectare ofmuch importance. 
It is therefore necessary to give meteorological data, partly collected 
by the writer in the neighbourhood of Tucson, Arizona, which is 
under much the same weather conditions as the neighbourhood of 
Los Nogales, Mexico. 
Air Temperature, 5 c.m. above the ground at Tucson, Arizona-— 
Altitude 723 metres, 
Month. 
Temp, in 
Sun °C. 
Temp, in 
Shade °C. 
Month. 
Temp, in 
Sun °C. 
Temp, in 
Shade °C. 
January 
18-5 
12-5 
June 
350 
25*0 
February 
24*7 
150 
July 
38*2 
26*1 
April 
30“ 2 
20*1 
August 
41-5 
30-2 
May 
27-3 
22-0 
September... 
390 
25-5 
The annual rainfall in different sections of Southern Arizona, 
varies with the altitude, as may be seen from the following figures ; 
the rainy season at lower elevations is in winter. 
Name of 
Place. 
Elevation 
in Metres. 
Summer 
rain in 
Millimetres. 
Winter rain 
in Millimetres. 
Annual 
rainfall in 
Millimetres. 
Yuma 
45 
10 
51 
78 
Maricopa ... 
351 
45 
78 
143 
Casa Grande 
711 
40 
71 
130 
Tucson 
723 
134 
114 
274 
Oracle 
1440 
182 
215 
449 
