Periodical Literature 89 



3. It forms an extraordinarily straight shaft contrasted with 

 the noted sabre-like form of the Alpine variety. 



4. The bole of the Sudetic form shows little taper as compared 

 with the tapering alpine form, which also has a thicker bark. 



5. Heart formation proceeds sooner and more rapidly in the 

 Sudetic form, which is explainable by the less broad crown; and 

 this difference in crown development also explains the shaft form. 



6. The wood of the Sudetic form is heavier throughout, and 

 especially in the lower bole, but the decrease in weight from base 

 to top is more rapid. 



7. The volume production in single trees does not differ essen- 

 tially, but in stands the possibly denser position of the Sudetic 

 form should furnish a larger product. 



Studien tiber die Alpen- und Sudetenldrche. Centralblatt fiir das gesammte 

 Forstwesen. May-June, 1914, pp. 171-184. 



Zon points out that the seasonal grouping 



Climatic of temperature data is without significance 



Conditions to the proper understanding of, plant life. 



and Plant This might apply even to Mayr's four 



Activities growing months, the "tetrahore." The 



period of vegetative activities varies from 



five months to twelve months in the United States, as shown by 



Zon's map. The author might have gone a step farther and said 



that different species of trees have different periods of cambial 



activity, of food manufacture, food storage and digestion. Until 



we know the lengths of these periods in each case, we shall not 



properly understand the adjustment of a species to its temperature 



conditions. 



The relation of plants to heat is expressed not in the absolute 

 amount of heat required by them, but in a certain combination of 

 time and heat. At its optimum of temperature each plant requires 

 a definite number of days in order to complete a given phase of its 

 development. Any deviation from the optimum in one or another 

 direction will lengthen this period. Based upon these facts, the 

 author suggests the following grouping of temperature data. 



Freezing days, with a daily average of 32° F. or less, (a) Freez- 

 ing days without thawing; (b) freezing days with thawing. Cold 

 days, with an average daily temperature ranging from 32° F. to 



