6 BOTANICAL GAZETTE [JULY 
Another important physiological study of soil-moisture relations 
is ALWAY’s (1) investigation of the relation of non-available water 
to the hygroscopic coefficient. He has shown that some kinds of 
plants can remain alive for a considerable time after growth ceases 
from lack of moisture, while others die rather promptly. This is 
doubtless one of the main differences between xerophytes and 
mesophytes. In the case of desert perennial legumes, life was 
maintained even after the soil moisture had fallen slightly below 
the hygroscopic coefficient. These results emphasize the need of a 
measure for the surface force of soils which can be expressed, or at 
least interpreted, in biological rather than physical terms. 
There have been few observations on the relation of seeds to 
soil moisture. BoGpDANOFF (3) studied the relation of germinating 
seeds to soil moisture, and presents many interesting facts. WHIT- 
NEY and CAMERON (36) noted the fact that a quantity of cowpeas 
whose hygroscopic moisture amounted to’ about 14 per cent, when 
mixed with an equal quantity of soil which contained 15 per cent 
of water, took up 12.1 per cent of their own weight, leaving only 1.3 
per cent of moisture in the soil. That is, the soil was practically 
air-dry. In the paper referred to (33) I have shown that the initial 
internal force of air-dry seeds is little short of 1000 atmospheres; 
if this condition be general among air-dry seeds, the behavior of 
the cowpeas can easily be understood. The relation of seeds to 
soil moisture and vapor pressure will be considered in more detail 
later. 
III. Materials and methods 
Marertat.—The Xanthium seeds used in the experiments dis- 
cussed in the following section were secured from plants raised on 
the experimental grounds of the University of Kansas in 1913. 
Originally all of the seeds planted were from a single plant of 
X. pennsylvanicum Wallr. The 119 plants obtained were very uni- 
form in all their obvious characters, and since it has been shown - 
(34) that the intermingled local types of Xanthiwm are practically 
isolated by differences in the blooming time of each species, the 
seeds may be considered as having come from a fairly pure line. 
This was thought desirable in order that the individual variations 
of the seeds might be reduced to a minimum, and that consequently 
