NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 221 



The angiosperms may be grouped in order, low: SA, TTCw, and 

 TTM ; medium : WAp ; high : WCh, WPe, WPl, TTL, and TTAp. 

 In general, the pines have a low percentage of agreement among their 

 branches whereas the Arizona cypresses are inclined to a high per- 

 centage and yet they grow in essentially the same environment. The 

 hardwoods from the same general locality vary in agreement from 6 

 to 100 percent. Those trees from the vicinity of Washington, D.C., 

 are intermediate between the above extremes. 



Insofar as present material is concerned, the generalized differences 

 among species appear to outweigh the differences from tree to tree 

 within a species. There remain, however, many differences between 

 trees of the same species growing near each other which could well be 

 ascribed to microsite factors or to individual growth form. One is in- 

 clined to correlate the disagreement among branches with the extreme 

 forest-border conditions, and this may be legitimate. But the case of 

 the Washington trees makes investigation under other conditions 

 necessary. 



Our third question (p. 205) reads: "Is there a similarity among 

 branches according to the year ?" In other words, is there a tendency 

 toward unity or multiplicity of the annual increments during certain 

 years? Because of the forest-border conditions, perhaps a better 

 phrasing of the question is : "Are there certain years with a tendency 

 away from the multiplicity now known to be typical of such an en- 

 vironment?" 



Table 159 gives the ratio of single to multiple branches in actual 

 numbers as well as pure ratio. It also gives the number of each type 

 of branch for those species with branches sufficiently numerous to 

 have significance (small numbers in left-hand boxes = total number of 

 branches involved). 



The most striking fact apparent at once is the great tendency toward 

 multiplicity in all years, a fact also emphasized by previous tables. Of 

 all years with a significant number of branches in the calculations, 

 1940 has the highest ratio of multiple branches — i single to 7.4 mul- 

 tiple. Other years with a high ratio of multiplicity are 1935, 1936, and 



1944. The years 1938, 1939, and 1942 have moderate ratios whereas 

 1937, 1 94 1, 1943, and 1945 have low ratios, considering the over-all 

 tendency toward multiplicity. The lowest ratio of the series of years is 



1945, with I single to 0.6 multiple. If these ratios are plotted, it will be 

 seen that there is a slight degree of repetition every 4 years and the 

 very slightest suggestion of a repetition at about 10 years. Because of 

 the great variations year to year (table 159), regardless of species, it 

 seems clear that they are due to variations in habitat factors. 



