tg1t] DACHNOWSKI—CRANBERRY ISLAND 147 
not one of relation to a single factor. In the interrelation of con- 
ditions, the real limiting factor to an increase in functional activity 
is not evaporation or temperature, but the toxicity of the sub- 
stratum. This fact reveals itself only in experimental tests. 
Toxicity comes markedly into play when the amount of water 
available for absorption has reached a stationary value, through 
Fic. 8.—Experiment station in the cranberry-sphagnum association; in the fore- 
ground stunted growth of Rhus Vernix; photographed July 31, roto. 
the activity of bacterial organisms and other processes. In the 
field it is very difficult at times to decide which features of the 
vegetation are to be correlated with low atmospheric humidity, 
which with variations in temperature and light conditions, or with 
other factors cooperating at the same time. It is obvious that 
each in its turn may play the part of a limiting factor, for growth 
and transpiration are very susceptible to variations in either of 
these conditions. But in the laboratory the extended experi- 
