ENVIRONMENT 95 



to the summer rains and displays additional growth. As a rule, near 

 Flagstaff this late growth is very much less in width than the spring 

 growth, usually between 10 and 20 per cent, rarely going to 30 per 

 cent. When it is more than 15 per cent, it begins often to show a 

 double effect, with its central part lighter than the red on each side. 

 In extreme cases this autumn growth actually gets back to the color of 

 spring wood and the growth becomes nearly white, thus separating 

 off an extra red ring that is rarely hard to distinguish from the annual 

 autumn red ring. The distinctive feature is that the false ring fades 

 gradually on both sides, while the true autumn ring fades gradually 

 on the inside but ends abruptly on the outside. 



Doubling and locality — The trees near Prescott show an extra- 

 ordinary number of extra rings, usually easily distinguished by the 

 criterion just mentioned. Some trees there have extra rings unusually 

 small and sharp and separated by very white tissue. Such rings are 

 more difficult to recognize. Sometimes there was more than one 

 false ring. In such cases it is evident that the storm is very important 

 to the tree. At that elevation, 5,200 feet, the rainfall is much less 

 than at Flagstaff, and each rainy season is more nearly a series of 

 isolated storms. 



The soil on which these Prescott rings grew is a disintegrated 

 granite which forms a very efficient reservoir, holding abundant 

 water with little leakage. The top of Mount Wilson is very similar 

 in type of soil, though not in climate, for it has a single rainy season in 

 winter. Double rings are practically unknown there. At the lower 

 levels of the Santa Rita Mountains near Tucson the soil and also the 

 climatic conditions are again similar to those at Prescott. The trees 

 there depend on summer rains even more than at the northern moun- 

 tains and the doubling character is more conspicuous and bothersome. 

 Thus it is seen that doubling is a local climatic effect. 



Doubling and age — Doubling is far more conspicuous in the earlier 

 or "youth" rings of a tree when the trunk is rapidly increasing in 

 size. These youth-rings are larger and less sensitive than the later 

 rings. Of course, it is more apparent in large rings, and any tree which 

 grows rapidly is more likely to show it. However, without specially 

 investigating the point, one is inclined to think that young trees, 

 being less sensitive than mature ones, are a little more certain to 

 continue their growth into autumn and so do have more doubling 

 than mature trees. This could be tested by the dendrograph on 

 properly selected trees. 



Doubling and summer rains — Since in double rings the space 

 between the false ring and the outside of the real autumn growth is due 

 to summer rains, it seemed possible that this segregated autumn 

 growth might give a measure of the summer rains. This was called 



