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Histological Studies in the Artemisia Formation 25 
force over their water-content and yield up water to neighboring 
tissues only in time of extreme need is not known. In many cases 
a latex-like substance is present. Parkin in his studies on Cey- 
lonese latex-bearing plants came to the conclusion that the chief 
function of latex is water-storage. We can not attempt to ex- 
plain the full meaning of water-storage cells until the exact physi- 
cal and chemical nature of the contents of such cells is deter- 
mined. Because of its evident relation to xerophytic vegetation 
the problem demands a speedy solution. 
SUMMARY 
The chief points reported in this paper may be summarized 
briefly as follows: 
1. The Artemisia formation is an arid, xerophytic formation. 
2. The average holard is 13.3 per cent. The humidity is low 
and the temperature is high. 
Humidity and temperature show great diurnal fluctuations. 
. Plants are found in a holard as low as 4 per cent. 
There is very little variation in thé vertical distribution of 
soil moisture in the sandy soils of the formation. 
6. Root systems are superficial; and the absorbing area of the 
roots is greatly augmented by the increased number of 
rootlets and root hairs. 
7. Artemisia tridentata is rather stable in the matter of leaf 
structure. The mean annual increment of the species .is 
very low. 
8. The plants of the formation as a whole are stable. 
g. The presence of a close covering of epidermal hairs does not 
necessarily preclude cuticularization. When one is lack- 
ing the other is usually present. 
to. Nuclei are normally present in the esiaereesl cells of the 
plants found in the Artemisia formation, 
11. There is a great predominance of palisade over sponge tis- 
sue; the chlorenchyma is very compact, the intercellular 
air spaces being reduced to mere lines in many species. 
12. “Water storage tissue” is present in nearly all of the species 
investigated. ; 
435 
