222 



BOTANICAL GAZETTE 



[SEPTEMBER 



is more important, during three different weeks of each year the 

 supply falls almost to or actually below the wilting coefficient. It is 

 thus an association in which the growth-w r ater fails repeatedly, 

 and consequently with respect to its water supply is decidedly more 



Fig. 18. — Graphs showing the range of soil moisture in the pine dune for 191 1; 

 the heavy line at 7 . 5 cm. and the light line at 25 cm. depth; wilting coefficient repre- 

 sented by broken lines. 



Fig. 19. — Graphs showing the range of soil moisture in the pine dune for 191 2; 

 the heavy line at 7. 5 cm. and the light line at 25 cm. depth; wilting coefficient repre- 

 sented by broken lines. 



xerophytic than the cottonwood dune. Reasons for the failure of 

 the water supply may be found in the comparative absence of the 

 conserving mulch of dry non-conducting soil, and in the much larger 

 demands made by the denser stand of vegetation. The plentiful 



