122 BROOKLYN BOTANIC GARDEN MEMOIRS 
It is of course impossible with any degree of certainty to determine 
from these data why the trees in one nursery suffered severely, whereas 
those in the other nearby location were but slightly affected. How- 
ever, if one were to hazard a guess he might say that it was due to 
excessive transpiration, the chief immediate factor being differences 
in wind velocity. A similar result has been obtained experimentally 
when trees which have been shaded were suddenly exposed to sun and 
wind. The next day they showed “tip-burn”’ of the pathologists, 
or a ‘physiological disease’? whatever that may be. One fact is 
clear. The death of the leaves was not due to the average conditions 
prevailing during the summer. 
During the winter months the average soil temperatures in the 
nursery for depths of three, six and twelve inches were 35.8° F., 37.9° 
F., and 38.5° F. Each figure is the average of 2,100 readings taken 
every two hours from the record made by a Friez machine. They 
show little differences in temperature at the various depths given. 
When, however, the record is examined for critical periods it is found 
that probably the most important season was that from March 28 
to April 18. During this period of 21 days the soil three inches deep 
froze and thawed sixteen times, at six inches, nine times and at twelve 
inches four times. Similar data collected in the adjacent forest 
showed that the soil both at six and at twelve inches thawed only 
once. We have no experimental data which determine the physio- 
logical meaning of these facts, but it is easy to surmise that in studying 
the effect of soil temperatures on plants we will not go far afield if 
we study carefully conditions obtaining during these critical periods 
as well as indeed full more than the general averages for the entire 
winter. 
Numerous examples could be given to show that averages extending 
over long periods for humidity, sunshine, wind, air temperature, etc., 
not only explain little but, on the other hand, conceal the essential 
facts. In all study of the relation of weather conditions to the develop- 
ment of plants the importance of critical periods in the environment 
must be taken into consideration. 
