SITUATION AND OCCURRENCE 19 



effect of the sun and wind by the removal of 

 the crop overhead. Their leaves are undevel- 

 oped, and cannot perform the necessary assim- 

 ilative functions under the changed conditions; 

 nor are the plants themselves adapted to resist 

 the drying action of the sun and wind. 



A tree produces a full crop of seed when by 

 the processes of its growth it has stored up in 

 its cells a sufficient supply of digested plant 

 food to furnish raw material for the fruit, when 

 its blossoms succeed in ripening, and when other 

 conditions, less clearly understood, are also 

 favorable. When these things happen to a con- 

 siderable proportion of any species in a forest at 

 the same time, a seed-year is said to take place. 



From a study of several hundred seedlings it 

 appears that the White Pine bears seed on an 

 average once in from 3 to 5 years, but that no 

 regular interval exists between seed-years. The 

 occurrence of a heavy crop of seed in 1892 is 

 recorded in the numerous seedlings of that year 

 which carpet the openings and the edges of the 

 woods. A great deal of seed ripened about 1840, 

 for much of the second-growth Pine in Penn- 

 sylvania, as well as in a number of other States, 

 dates from that time. It is only possible, from 

 our limited observations, to say that full seed- 

 years occur only at long intervals. 



