119 



and branches of most woody plants. Sometimes it is possible to 

 trace the annual growth back through twenty five or thirty years 

 by reference to the branching system. Pinus ponderosa is a 

 favorable subject for study in this particular. At the top of the 

 leader a strong bud is formed toward the conclusion of the growing 

 season. Close to the base of this bud are formed several lateral 

 ones, the subsequent development of which produces a whorl of 

 branches. As other lateral buds are usually lacking along the 

 shoot of the season, it follows that each circle of branches marks 

 the limit of a season's upward growth of the main stem. xA.s the 

 internodes, or spaces between successive circles of branches, do 

 not increase in length after they are first formed, the height 

 increment of all the past seasons can be determined, so far as one 

 is able to identify the distinct whorls which mark the conclusion 

 of each year's growth. 



That the length of the internode in the western yellow pine 

 growing in dry situations is an expression of conditions affecting 

 growth, not in the present, so much as in the preceding season, is 

 now shown with remarkable clearness by the trees of this region.* 

 Some of the facts here set forth were observed on an island in 

 Flathead Lake during the summer of 1913. Trees were growing 

 at altitudes varying from four or five to one hundred feet above 

 the level of the lake, and in the more open places among the 

 mature trees many younger individuals were present varying 

 from four to fifteen feet in height. Of many of these it was 

 observed that the internode for 191 1 was considerably shorter 

 than that for 1910. Reflecting upon the fact that the season of 

 19 10 throughout the whole region was exceptionally dry, and 

 that of 191 1 much more favorable, the question arose as to the 

 apparently contradictory evidence of the internodes. Why 

 should the growth be less in a season supposedly more favorable, 

 and greater in a season obviously less advantageous? 



It is evident that the growth in length in each season is not 

 directly affected in this region by the lack of rain in July and 

 August, since the elongation of the shoot is practically ended 

 before the dry season sets in and before the moisture from winter 



* Pfeffer, W. Plant Physiology, Vol. I, p. 603. 



