992 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 u, 1,937 wu, and 3,278 uw. 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 
} Whitten, J. C. pe protection of the peach. Missouri Agr. Exp. Sta. 
Bull. 38: 140-164. 1897. 
