34 
A. G. Tansley. 
More or less intermediate between the two formations stands 
the wire-grass association dominated by Aristida longiseta. This 
occurs on Great Plains soils which are rather more sandy than 
those carrying the typical short grass association, but is included 
by Dr. Shantz in the short grass formation. 
For a full and most interesting description of the vegetation 
whose characters have been briefly outlined, the reader should 
consult Dr. Shantz’s bulletin already referred to. 
The ordinary “ dry farming ” practice in this region is to fallow 
the land in alternate years. The surface-layer of dry soil acts as a 
dry mulch and preserves much of the water of precipitation falling 
during the fallow year in the deeper layers of soil. Thus a much 
better crop is obtained during the next year. The proceeding is 
however exceedingly risky, because a year of extreme drought may 
upset the calculation, and it is perhaps doubtful if even over a series 
of years, much advantage is obtained. “ Dry farming ” in this 
region scarcely pays except in association with ranching. 
Of abundant ruderal weeds the so-called Russian thistle ( Salsola 
pestifer ) is the most conspicuous. This and some other species 
(e.g., AniarautJius grcecizaus ) belong to the class of “ tumble weeds,” 
i.e ., they easily become uprooted or broken off and blow about the 
plains where they may often be seen caught in lines at the wire 
fences. Salsola shows very beautifully, when growing on the edge 
of a crop bordering on fallow, the marked effect of differences of 
water-content of the soil in determining root-growth. The roots 
of such a plant are all bent out, sometimes at right angles to the 
vertical axis, away from the crop and towards the fallow, and such 
plants are much larger and more luxuriant than those growing 
actually among the crop-plants. Similarly on the edge between a 
crop and an area of short grass, the Russian thistles have their 
roots bent towards the crop where the competition for water is less 
severe than in the short grass area. These phenomena give an 
excellent idea of the very severe competition for soil water that 
must go on in any closed association in this region. 
Leaving Akron in the late afternoon of August 11th, the party 
travelled to Denver and thence southward to Palmer Lake on the 
edge of the Rocky Mountain foothills. Before reaching Denver an 
irrigated region is entered, a region in which a supply of river water 
is available for irrigation; and here the country is much more closely 
occupied by crops, and these are, on the whole, much more luxuriant 
than in the “ dry farming” region further east. All along this part 
