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. 



