6 BOTANICAL GAZETTE [july 



Another important physiological study of soil-moisture relations 

 is Alway's (i) 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. Bogdanoff (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 



Material. — 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 

 A*, pcnnsylvanicum 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 Xanthium 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 



