212 



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, (ft) 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.' Data were secured to 

 bear out the foregoing statement as early as May lo, 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 10 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. 



