18 CLIMATIC CYCLES AND TREE-GROWTH. 
that the use of the year beginning November 1 at Flagstaff and 
September 1 at Prescott gave the closest agreement between growth 
and rainfall. At Flagstaff the majority of the trees came from a thin 
clay soil derived in place from decomposed lava, and so there was little 
depth for the storage of moisture. At Prescott the sections of group 5, 
shown in the solid line of figure 7, came from trees growing in a porous 
soil of decomposed granite in a rather flat depression with reaarded 
drainage, so that conservation would have a greater influence. Perhaps 
this explains why the year beginning September 1 gives the best results. 
In the region of the great sequoias nearly all the precipitation in the 
mountains (and quite all in the valleys where comparative rain records 
are found) comes in the winter months. For these trees, therefore, 
the winter precipitation is compared with the growth for the succeeding 
year and the month of beginning annual means is in the autumn. 
THE TIME OF YEAR OF RING FORMATION. 
Among the problems connected with the relation of the growth of 
trees and the amount of rainfall, one of the most interesting was sug- 
gested by Director R. H. Forbes, formerly of the Arizona Experiment 
Station. This was to determine the time of formation of the red or 
autumn portion of the rings and the causes for the formation of double 
rings, which were very numerous in the Prescott group. It seems 
evident at once that the growth of red cells is related to the decreased 
absorption of moisture as winter approaches. A number of tests were 
made on the Prescott group. The first was designed to determine 
the character of the rainfall in the years producing double rings. The 
half-dozen most persistent cases were selected and in each of these the 
red ring was found double in the following number of cases: 4 out of 
10 in 1896; 5 out of 10 in 1891; 7 out of 10 in 1881; 4 out of 10 in 1878, 
1872, and 1871. The average width of all the rings was 1.55mm. The 
mean rainfall by months for the years above selected was found and 
is plotted in the solid line of the upper diagram of figure 1. Six other 
rings showing one double in 10 trees in 1898, but no doubles in 1897, 
1885, 1884, 1876, and 1874, and averaging 1.54 mm. in thickness, were 
then selected and the curve of rainfall by months for the year during 
which they grew has been plotted as the upper dotted line in figure 1. 
In each curve the 6 months preceding and the 2 months following the 
year are included. The curves seem to indicate clearly that the chief 
cause of doubling is a deficiency of snowfall in the winter months, 
December to March. This appears to mean that if the winter pre- 
cipitation is sufficient to bridge over the usual spring drought, the 
growth continues through the season, giving a large single ring which 
ends only in the usual red growth as the severity of winter comes on. 
If, however, the preceding winter precipitation has not been entirely 
adequate, the spring drought taxes the resources of thetree and some red 
tissue is formed because of deficient absorptionin the early summer before 
the rains begin. When these rains come the tree continues its growth. 
