346 BOTANICAL GAZETTE [DECEMBER 
can and British ecology concerns itself with the measurement of 
such factors as soil moisture, soil chemistry, temperature of both 
soil and air, evaporating power of the air, and intensity of sunlight. 
With the cooperation of ecological plant physiologists, a number of 
more or less satisfactory methods for the measurement of these 
factors have been evolved. Among the many workers on the prob- 
lems of soil moisture, BR1Gcs and SHANTz (3) may be cited for their 
development of the wilting coefficient concept, Br1ccs and McLANE 
(2) for the moisture equivalent idea, ALway (1) for the hygroscopic 
coefficient, and Livincston and Koxersu (11) for the invention 
of the so-called soil points. Thermometric data were among the 
earliest to be gathered, although they are still among the least 
satisfactorily interpreted; special citations appear to be super- 
fluous. Exception might be claimed for the “‘life zone” idea best 
developed by Merriam (15), but this is regional-climatic rather 
than local and topographic. 
Modern ecological work in the measurement of the evaporating 
power of the air dates chiefly from the re-invention and populariza- 
tion by LivincsTon (8, 9) of the porous cup atmometer. From 
time to time numerous attempts have been made to develop photo- 
graphic and other methods for a field study of sunlight intensity, 
but none of them has been very satisfactory to students of plant 
activities. PULLING (16) gives a concise review of the work in this 
field. About the only instrument at present used by anyone except 
its own inventor is Lrvincston’s radio-atmometer (10), which 
obtains an approximate measurement of the effect of direct solar 
radiation on evaporation from a free water surface. Other methods 
for the measurement of ecological factors have some local vogue, 
but those here outlined are the ones most frequently used. 
So widespread has been the practice of factor measurement, and 
so many the workers, that a complete review of the literature would 
be impossible in this connection, and in view of the extensive litera- 
ture cited in such standard works as those of Livincston and 
SHREVE (12), and of CLEMENTS (14), may well be omitted. 
An important consideration in the development of a successional 
series, but one that is not always given the prominence it deserves, 
is the fact that the determining conditions that permit or bar the 
