l86 BOTANICAL GAZETTE [march 



accompanied by weather conditions which stimulate both evapora- 

 tion and transpiration, and so favor a reduction of the soil moisture 

 content. These favoring conditions are high temperature, low 

 atmospheric humidity, high wind velocity, and a high degree of 

 insolation. Even in a semi-arid region several years may pass 

 without the concurrence of the necessary conditions, while in 

 humid regions such intervals are of still greater length. 



The results of the greenhouse experiments of Briggs and 

 Shantz (6) would suggest that after periods of extreme drought 

 the prairie subsoil might be expected to show a moisture content 

 which either approximated the so-called wilting coefficient (1.47 

 times the hygroscopic coefficient) or which was somewhat lower 

 than the former value but bore no distinct relation to the latter. 

 In the opinion of these authors the wilting coefficient "practically 

 marks the cessation of growth," and after this point has been 

 reached the soils continue to lose water through the tissues of the 

 plants, even after they are dead, the final moisture content of the 

 soil being as low as though the soil and air had been in direct 

 contact. However, pot experiments of any kind, and especially 

 those employing shallow vessels, appear ill adapted to answer the 

 question as to how dry a particular soil may become under field 

 conditions. Accordingly the data obtained in the field at such times 

 as when the weather conditions have been favorable to an extreme 

 reduction of the subsoil moisture should prove of especial interest, 

 provided they are accompanied by determinations of the hygro- 

 scopic coefficient or wilting coefficient of the soils. 



In regions of winter rains and summer droughts, such as Cali- 

 fornia, one may safely count upon the continuance of hot rainless 

 weather with clear skies for many weeks after the dry season has 

 once set in, but in those with summer rains one never knows when 

 to make preparations for studies that are dependent upon extreme 

 brought conditions, and it may happen that, after all arrangements 

 have been completed, several years may pass before weather con- 

 ditions favoradle for the work occur. For this reason data on the 

 soil moisture content during unusual droughts in humid regions 

 are most apt to be secured in the course of some less specialized 

 investigation. 



