SOIL MOISTURE AND WILTING 



125 



of the dry soil. A rough mechanical analysis of each soil is given 

 in the following table. 



TABLE 1 



Analysis of clay-loam and sand 



The experiments will now be described, each of the four species 

 used being separately considered. 



Martynia Louisiana Mill. This species grows spontaneously, 

 and in large numbers, near the Desert Laboratory during the 

 moist mid-summer. It also occurs earlier, while the dry season 

 is at its height, in irrigated places. This plant is therefore well 

 suited to stand the high evaporation rate of southern Arizona. 

 Similar plants of this species, which had been grown side by side 

 in the open, were placed under the various conditions previously 

 described and allowed to wilt. The percent of soil-moisture at 

 the time of wilting, for each case, is given in table 2. 



An examination of table 2 shows that there were considerable 

 variations in the soil moisture content at wilting, even when 

 this occurred under the same rate of evaporation. This was 

 probably due in part to differences in the individual plants, and 

 in part to the crude methods here resorted to. Considerable 

 difficulty was experienced in deciding when one plant had reached 

 the same stage of wilting as another. This was particularly 

 true of Martynia and of Physalis, (the next plant to be con- 

 sidered) and may have introduced considerable errors. Another 

 cause for error lies in unequal packing of the soil in the several 

 pots. Despite these irregularities it seems clear, however, that 

 there is, in general, a very close relationship between the soil 



