6 Raymond John Pool 
proceeded for a few centimeters in crevices and soft places in the 
rock in which soil has accumulated to a sufficient degree to fur- 
nish a foothold for plants. On such hillsides Artemisia tridentata 
was abundant up to 2,830 m. (8,500 ft.). By the exercise of its 
soil-forming function, by the addition of litter and humus to the 
originally scanty supply, considerable soil has been accumulated, 
and the plants now cover rather wide areas through which yellow 
pine (Pinus ponderosa) is also scattered. The soil here, in con- 
trast to that in the south, is very shallow, as well as rocky and 
gravelly. In it soil samples were taken with considerable 
difficulty. 
From 108 water-content determinations for the region studied 
it was found that the average physical water-content or holard 
for all depths in the formation was 13.3 per cent. The extremes 
were 27.9 per cent near the edge of an irrigation ditch at a depth 
of 8 dem., and 4.0 per cent in the open homogeneous sandy soil 
at a depth of 2 dem. in the absorbing area of a species of Atriplex. 
This minimum is very low. Probably it can be explained only 
by reference to the structure of the soil. As above stated, the 
soil here, Fort Garland, is a fine homogeneous sand with a very 
fine surface mulch of about a centimeter in depth. Now it must 
be that the echard, or the non-available water-content here is ex- 
tremely low, probably less than 1 per cent. It is regretted that 
no determinations were made on this point, for certainly such 
would have been enlightening. The very great development of 
rootlets and root hairs was, however, noted for the above plants. 
This fact, together with the very hairy leaves containing much 
storage tissue, would indicate that here is a plant which is excel- 
lently adapted to a very low holard and in a.way to prevent 
desiccation by excessive transpiration. 
Another point brought out in the water relations of xerophytic 
plants during these field observations is that there was very little 
variation in the vertical distribution of soil moisture, especially 
in the homogeneous sandy soils of the formation. This condition 
is accompanied by a rather superficially developed root-system. 
™n fact, the absorbing area for most species of the herbaceous 
associates of the Artemisia was found to be at a depth of about 
416 
