TYPES OF STEMS. 



367 



are distributed along the stem. To the first type belonfj; the 

 pines, N(jrway spruce, Douglas pruce, etc. The white pine is 

 an exquisite example, and in 

 young and middle-aged trees 

 shows the style of branching to 

 very good advantage. The 

 branches are nearly horizontal, 

 TOth a shght sigmoid graceful 

 curve, while towards the top the 

 branches are ascending. This 

 direction of the branches is due 

 to the light relation. The few 

 whorls at the top are ascending 

 because of the strong light from 

 above. They soon become ex- 

 tended in a horizontal direction 

 as the main source of light is 

 shifting to the side bv the shad- 

 ing of the top. The ascending 

 direction first taken by the upper 

 branches and their subsequent turning downward, while the ends 

 often still have a shght ascending direction gives to the older 

 branches their sigmoid curve. 



The young vernal shoots of the pines show some very interest- 

 ing growth-movements. There are two growth periods: (i) the 

 elongation of the shoot, and (2) the elongation of the leaves. 

 The elongation of the shoot takes place first and is completed in 

 about six weeks or two months' time. The direction of the 

 shoot in the iirst period seems to be entirely influenced by geot- 

 ropism. It grows directly upward and stands up as a very 

 conspicuous object in strong contrast with the dark green fohage 

 of the more or less horizontal shoots. When the second period 

 of growth takes place, and the leaves elongate, the shoot bends 

 downward and outward in a lateral direction. 



The rate of growth of the pines can be very easily observed 

 since each whorl of branches (between the whorls of long shoots 



a] tyfie of larcli. 



