23° 



BOTANICAL GAZETTE 



[SEPTEMBER 



in table III, and especially as expressed in the sixth and seventh 

 columns, which contain the mean percentages of growth-water and 

 the comparative amounts, the beech-maple forest being taken as 

 a standard and its mean growth-water represented by ioo. In the 

 five associations that form the succession, the mean growth-water 

 for midsummer of the two seasons will be found, if we except the 

 cottonwood dune, to form a progressive series, the most mesophytic 

 association having the largest amount. This was exactly what was 

 supposed to be the situation, but hitherto no quantitative data of 

 such moisture relationships have been available, and it has been 



TABLE III 



Wilting coefficients and mean percentages of growth-water in the various 



associations during the io midsummer weeks of 19ii-i912 



Association 





Cottonwood dune. . 

 Pine dune 



Depth in 

 cm. 



Wilting 

 coefficient 



Growth-water 



iqii 



1912 



f 



Oak dune 



1 



Oak-hickory. 

 Beech-maple 



Prairie 





7 



25 

 7 



25 

 7 



25 

 7 



25 



7 

 25 



7 

 25 



5 

 o 



5 

 o 



5 

 o 



5 



0.80 

 0.80 

 1. 10 

 1. 00 

 1. 10 

 0.90 

 9.0 



per cent 



2.2 

 2.6 









95 



5 



13 -5 







9-5 



5 



24.0 







21.0 



0.9 



0.8 



3-3 



5-5 

 30 



4.0 

 4 9 



Mean 



Com- 

 parative 

 am'ts 



per cent 



1.8 

 2.0 

 0.7 



1-4 

 i-5 



per cent per cent 



2. IS 



5° 



4.0 



4.0 



5-9 



iiS 



1 .27 



49 



26 



3-3 



4 4 



4-7 



29 



Ratio between 

 evaporation 

 and growth- 

 WATER 



II. 7 



75 



IOO 



107 



10.7 



9-37 



2.78 



1. 81 



2.91 



impossible to tell how much an association differs in its water con- 

 ditions from the preceding or succeeding association. As ha 

 already been stated, the cottonwood dune, with a larger and more 

 constant water supply than the two succeeding associations, must 

 owe its surplus to the conserving power of its dust mulch and to the 

 small outgo due to the paucity of its vegetation. 



The comparative amounts of growth-water indicate even more 

 clearly the relationship existing between the available water 

 supply of the associations, and should serve to emphasize the fact 

 that the progressive increase in the water-retaining power of the 



