34 BOTANICAL GAZETTE [JUL c 
except during very heavy rains. The total water content varies 
with the soil composition from 5-14 per cent., and all but 2-4 per” 
cent. is available to the plant. The crests are of a much looser 
‘soil, almost pure gravel in places, but generally mixed with clay and 
silt. The water content here is from 3-4 per cent. lower than on 
the Mesa top and the formation is much more open. The clay is 
closely packed and a large percentage of water which falls runs off. . 
The water content is usually less than 17 per cent. and is often as 
low as 13 per cent., while the non—available water seems to vary 
with the plant and the slight differences in the amount of foreign’ 
substances in the clay from 8.5-10 per cent. After a rain the soil 
is easily baked to form a hard crust, and when the soil becomes 
dry, especially during winter, it cracks open to a considerable 
depth. 
The preference which certain plants show for north and wes 
slopes is easily explained by a glance at the sunshine record for ull : 
and July. The morning sun heats the east slopes, and as it rises 
higher the south slope is strongly heated. This hastens trans 
tion and water loss. The afternoon is cloudy and very likely rainy 
The west slope is consequently not strongly illuminated, the soil ' 13 
temperature is from 2—5° C. lower than on the east and south slopes 
and the water content is from 1.5-5 per cent. higher. It is the leas 
xerophytic of any situation in the formation. 
AESTIVAL ASPECT. 
Habitat.—Physical water: in clay 10-13 per cent.; in gra 
2.5-4.5 per cent. Non-available water: in clay 8.5—-10 per cent. 
in gravel 1.5-2.5 per cent. Available water: in clay 2.3-3- om 
to roo of dry soil; in gravel 1-2.18™ to roo8™. Rainfall 7. SF 
10°™; av. daily 0.137°™. Relative sree mean 51 per cent. 
Tange g-Ioo per cent. Wind 4.3545" per hour. Tempera 
mean 18.8° C.; mean max. 25° C.; mean min. 10.5° C.; range 2- 
36.6° C.; soil 18-29° C:; soil sisrhace to 55°C. Light duration 
per cent. Exposure eel cover open, soil clay to gravel. D 
tion July 15 to September 1s. 
Facts: Bouteloua oligostachya (Nutt.) Torr., B. hirsuta Lag., (Andropo! 
scoparius Mich.), (A. furcatus Muhl.), (Calamovilia longifolia [Hook.] Hack 
