292 KNUDSON : CAMBIUM DEVELOPMENT IN AMERICAN LARCH 



What factors operate to cause growth inception in a particular 

 part of a tree? Robt. Hartig* believed that temperature was the 

 most important factor, consequently insolation, temperature of 

 the air and of the soil moisture, and thickness of the bark are 

 the essential factors which determine the region of first diameter 

 increase. No doubt these factors are important, as considerable 

 evidence indicates that in old trees diameter increase is delayed 

 at the base of trunk where insolation is poor and the bark is thick. 

 In young trees, however, these are not the only factors. In the 

 larch trees of 13 years of age the diameter increase did not begin 

 first in those regions with the thinnest bark and best insolation. 

 The thickness of the bark at the apex, middle and base of the 

 tree A was respectively 596/1, i,937/j and 3,278 ^. So also in 

 the isolated tree E the inception of diameter increase did not occur 

 in the parts of the tree best insolated and with the thinnest bark, 

 but rather in the thicker barked and more poorly insolated parts 

 of the tree, namely the middle and basal regions of the trunk. In 

 the individual branches, however, growth in diameter began in the 

 regions of thinnest bark and of best insolation. 



Whittenf has shown that the color of the bark may be impor- 

 tant in the time of growth inception of buds. The color of the 

 bark may be a factor in determining the region of diameter increase 

 in young trees of larch. The color of the bark of the apical part 

 of the trunk in spring is yellowish to greenish, becoming darker 

 towards the base. The darker color, because of its capacity for 

 heat absorption, may counteract the insulating effect of the thick 

 bark, consequently the diameter increase begins in the basal and 

 middle regions. The growth begins first in the middle regions 

 because the bark here is of the same color as that of the basal 

 regions and is only about half as thick. The fact that in the 

 branches and twigs of larch the opening of buds on the apical 

 regions is retarded is suggestive of the influence of the color of the 

 bark. The bark of the apical regions being of lighter color less 

 heat is absorbed, its temperature therefore is lower and the devel- 

 opment of the buds is slower. This is in agreement with the 



* Hartig, R., loc. cit. 



t Whitten, J. C. Winter protection of the peach. Missouri Agr. Exp. Sta. 

 Bull. 38: 140-164. 1897. 



