252 BOTANICAL GAZETTE [MARCH 
first two factors, therefore, may be eliminated from the discussion 
because both are at an optimum in the spring. 
The temperature of the cambial layer depends, on the other 
hand, on three factors: (a) temperature of the air, (b) temperature 
of the soil water, and (c) direct insolation. Which of the three is 
most potent in the awakening and rapidity of cell division has not 
been determined, because they are so closely related to each other. 
It would appear that the temperature of the soil water plays a 
prominent part in the awakening of growth because of the opening 
out of the phloem first near the base of the tree. Factors (a) and 
(c) would be entirely negligible here, or at least play a minor part 
because of the thick layers of bark. Growth in the spring begins 
before factors (a) and (c) could have reached any appreciable 
height, so that the heat derived from soil water is certainly potential 
in awakening growth. 
It is quite impossible to separate factors (a) and (c) and to note 
their effect in all trees. However, cuttings secured from the north 
and south sides of isolated trees at the same height often afford 
ample evidence of the effect of insolation.2 Data were secured to 
bear out the foregoing statement as early as May 10, 1913. The 
tree examined was a “Wolf” white pine on the south side of Fall 
Creek beyond Forest Home, N.Y., a suburb of Ithaca. The speci- 
men was 51 feet high, with a diameter breast height of 15.3 inches, 
and exhibited vigorous growth in spite of the poor soil conditions. 
At the date mentioned, tracheid formation had proceeded on the 
south side to the extent of 12-14 tracheids, while on the north side 
9 or ro tracheids had been formed. Lignification had not as yet set 
in, although all but the last 3 or 4 tracheids formed had apparently 
attained their ultimate size. That direct insolation is potent in 
the awakening of growth in trees is certain. However, one indi- 
vidual will present occasionally conditions the reverse of what 
would be expected. 
It follows from the preceding paragraph that the awakening 
and the rapidity of growth depends on three factors, two of which 
are at an optimum in the spring and may therefore be neglected. 
® Trees should be selected only from sites which are level, as trees growing on 
slopes are subjected to other factors which often overshadow the effect of insolation. 
