It appears further that if not only the winter snows are lacking, but the spring rains are unusually scanty, then the tree may close up shop for the year and produce its final red tissue in midsummer, gaining no immediate benefit from the summer rains. This appears to be the interpretation of the lower diagram of figure 1. Here the same 6 big
Fig. 1. — Effect of monthly distribution of precipitation on thickness of rings of growth; Prescott, Arizona.
doubles mentioned above are plotted, together with a selected list of 6 small singles particularly deficient in red tissues. They are, 1904 double once in 10, 1902 double once in 10, 1899 single, 1895 single, 1894 single, and 1880 double once in 10. In these it is evident that drought in the spring stops the growth of the tree. The double ring, therefore, seems to be an intermediate form between the large normal single ring, growing through the warm parts of the year, and the small, deficient ring, ending its growth by midsummer. This occasional failure to benefit by the summer rains probably explains why the Prescott trees do not show an agreement of more than about 70 per cent between growth and rainfall. It suggests also that the Flagstaff trees, which grow under conditions of more rainfall and have very few double rings, give a more accurate record than those of Prescott.Consistent with this view of the doubling is the condition of the outer ring in the Prescott sections collected by Mr. Hinderer. These trees