STEMS 



735 



theory maintains that salt particles carried in the spray account for 

 branch destruction. It is possible and even probable that both theories 

 are vaHd, the two factors supplementing one another. Apparently 

 favoring the salt theory is the relative absence of one-sided trees about 

 the Great Lakes, but apparently favoring the transpiration theory is 

 the frequent one-sidedness of trees near the " timber line " of mountains. 



Fig. 1056. — Trees of the Douglas spruce (Pseudotsuga mucronata) growing near the 

 sea ; note the relative absence of branches on the exposed (seaward) side, thus showing the 

 destructive influence of the sea winds ; San Juan Island, Wash. — Photograph by 

 O'Brien. 



Periodicity in stem development. — Periodicity in tree branches. — 

 The twigs of deciduous trees are made up of alternating dwarfed and 

 elongated portions, the leaf scars of the former being closely grouped, 

 while those of the latter are more widely separated (fig. 1057). A branch 

 begins as a lateral bud closely enveloped by scale leaves, and during the 

 first season growth commonly is slight; the following spring there is a 

 period of elongation accompanied by the fall of the scales, whose posi- 

 tion is marked by the closely grouped scars. Later, elongation ceases 



