1912] BRIEFER ARTICLES 513 
of shrubs and grasses, developing upon more or less rapidly moving 
dunes, possessing a very high rate of evaporation, exhibiting many 
strongly xerophytic characters, and almost entirely dependent upon 
vegetative reproduction for its maintenance. The almost complete 
absence of herbaceous undergrowth and the expanse of bare sand give 
it a desert-like aspect, but below the superficial layer of dry sand an 
abundant and unfailing water supply has been found 
From the beginning of May to the end of October 1911 duplicate 
samples of about 125 grams of soil were taken weekly at depths of 7.5 
cm. and 25 cm., the soil dried at 104° C., and the ratio of water to the 
dry weight of soil found to range from 2 to 8 per cent, an apparently 
very inadequate amount. 
Until recently no satisfactory means of relating such soil moisture 
determinations to plant growth have been available but the “wilting 
coefficient”? of Briccs and SHANTz3 now indicates the limit of soil water 
content above which growth must occur, although plants will live and 
continue to draw water from the soil much below this limit. Further, 
the same workers have shown that many plants differ very little in their 
wilting coefficients from the standard Kubanka wheat. As under 
ordinary conditions a water supply very little above that at which wilt- 
ing occurs is sufficient for some growth, the difference between the soil 
moisture actually present at any time and the wilting coefficient of the 
soil represents approximately the amount of water available for pur- 
poses of growth, and nee in the absence of a better term, may be referred 
to as ‘‘growth water.’ 
Following the methods of Briccs and SHANTz (loc. cit), the wilting 
coefficient of the dune soil was found to be 0.75 per cent at both depths, 
the absence of humus accounting for this similarity. Graphically 
representing this wilting coefficient and plotting the soil moisture deter- 
minations as graphs having the weekly intervals as abscissae and the 
percentage of water present in the soil as ordinates (fig. 1), it will be 
seen that the moisture present in the soil of the cottonwood dune is at 
all times more than double the wilting coefficient, or in other words 
there is always present at least twice the amount of water necessary for 
the growth of such a plant as wheat. Throughout the most arid por- 
tion of the season, namely the ten weeks beginning with the first of 
July, the surplus or growth water averages 2.2 per cent, showing that, 
3 Briccs, L. J., and SHantz, H. L., The wilting coefficient for different plants 
and its indirect determination. U.S. Dept. Agric., Bur. Pl. Ind. Bull. 230, 1912; 
also Bot. GAz. 53:20-37, 229-235. 1912. 
