1922] HARVEY—PINE FORMATION 31 
will adequately serve to indicate the critical ecological conditions. 
It was expected that some light would be thrown by such data 
upon the questions of the relation of evaporation to succession and 
upon the classical conception of the successional development of 
the formations of a region. In my paper already referred to, 
the data seemed to show that evaporation had no causal relation 
to succession. It was argued that the measurements of the 
ecological factors of any association are more largely a result of 
that stage of the succession rather than the cause of it. Specifically, 
the evaporation is less in the Big Pines formation than in the open 
herbaceous association, because of the greater protection in the 
former from factors enhancing water loss; the Big Pines forma- 
tion is the cause of lowered evaporation and not the result of it. 
The evaporation conditions of any pioneer association obviously 
represent the conditions which must be tolerated by the invaders 
from any higher ecological type of association. These invaders help 
to reduce the evaporation, and finally, as the next step in the suc- 
cession is established, it comes to possess its own evaporation ratio 
determined by its own canopy, shrubby, and herbaceous layers. 
SOIL WATER.—Soil samples were taken in two series at depths 
of 7.5 cm. and 25 cm. by means of a knife-blade trowel, and placed 
at once in friction top cans of 350 cc. capacity. The samples were 
collected on August 4, 11, 18, and 25. Analyses of the samples 
were completed within the following month. The moisture-holding 
capacity (MHC) was computed upon the basis of dry weight as 
well as upon the basis of equal volumes, according to HiLGarp’s? 
method. Each determination represents the average of the four 
samples for each station and series. The total field capacity 
(TFC) was computed directly upon the basis of dry weight. 
The wilting coefficient (WC) was calculated by the formula of 
moisture holding capacity — 21 
2.90(+ 0.021) 
which the MHC is determined by volume per cent. It is recog- 
nized that the WC data thus computed are only an approximation, 
as such data preferably should be based either upon experimental 
2 Hincarp, E. W., Soils. New York. 1906. 
3 Briccs, L. J., and SHantz, H. L., The wilting coefficient for different plants and 
its indirect determination. U.S. Dept. Agric., Bur. Plant Ind. Bull. 230; Bor. 
GAZ, §1:210-219. I9II; §3:20-37, 229-235. I9I2. 
; aR 
Briccs and SHANntTz2 .WC= 
