120 



snows and spring rains disappears from the soil, but indirectly 

 by the drouth in the months indicated, as affecting the supply of 

 reserve food in the buds and other parts. The supply of food 

 stands in direct ratio to the condition of several factors, among 

 which the amount of available moisture is very influential, during 

 the middle and later summer of the season next preceding. As 

 a lack of moisture affects adversely assimilative and other activ- 

 ities, a meager store of nutritive materials would be the result 

 of a dry summer, and would find expression the following year 

 in reduced growth in length of all the new shoots. Of course 

 this weakness of the buds would not occur in trees as a result of 

 diminished precipitation in situations providing adequate soil 

 moisture throughout the season, but only where drouth appears 

 early, as is usually the case on southern and western slopes. For 

 this reason those trees in the locality mentioned which grew 

 nearer the water level showed no such inequality in the length of 

 the internodes, and the same was found to be true in more favored 

 situations elsewhere. 



It is true, of course that assimilation can take place at all 

 seasons in evergreens, under proper conditions of temperature 

 and illumination. It has been shown that assimilation may 

 proceed in some plants at a temperature as low as — 40° C, and 

 that in pines and spruces the process is active at 3° to 5° C. In 

 the northern Rocky Mountain region, however, the temperatures 

 are considerably below 0° C. during most of the winter, and fre- 

 quently below — 40° C. The soil, moreover, is frozen to a depth 

 of several feet, resulting in a reduction of the water supply. In 

 this problem, however, it is chiefly a question of relative activity 

 and not of complete suspension of photosynthesis at any time, 

 while we are dealing with reserve material in the form of proteid, 

 not starch, nevertheless the synthetic activity of the tree, so far 

 as this material is concerned, is closely associated with starch 

 formation, according to the best evidence at present available. 



Seeking further evidence on the relation of the growth of 

 yellow pine to the distribution of rainfall, the writer made two 

 series of observations in the vicinity of Missoula, the results of 

 which are given in the accompanying tables. \\'here the forest 



