FROM SIMPLE LEAVES. 123 



branches thus concentrically disposed about the main trunk 

 are arranged not only symmetrically but bilaterally, the two 

 sides of each branch, like those of the leaf, being counterparts 

 of each other. This remark applies to branches with droop- 

 ing branchlets, as well as to those which spread out horizon- 

 tally from the sides of the branch. For the leaves which are 

 arranged concentrically about the trunk assume a bilateral 

 distribution on the branches in proportion to the increase in 

 the amount of their horizontal development ; and in the lowest 

 branches, whose growth is most horizontal, the leaves drop 

 from the under surface, or such as remain there curve round 

 to the sides of the branch for light, so that they are mainly con- 

 fined to its upper surface and two sides. Now it is plain 

 enough that the axilla3 of the leaves situated on the upper 

 side of the branches would be bad starting points for grow- 

 ing shoots ; and for this reason the upper side leaves remain 

 sterile. Therefore this bilateral arrangement of the branch- 

 lets or side-growths of the branches, is the natural result of 

 their horizontal growth and their overshadowed position, for 

 branches always develop their branchlets in those directions 

 where there is the most space and light. 



Again, in the Beech, Linden, Hazel, Elm, and Hornbeam, 

 this bilateral symmetry of the branches holds not only for 

 the ramifications of the secondary axis but for the whole 

 tree during the first years of its life. For these branches 

 proceed from buds which are developed by alternate, dis- 

 tichous, or two-ranked leaves; hence, in the Spring of the 

 second year they will necessarily lie in one plane, like the 

 beard of a feather. This is well seen in the second year's 

 growth of the Linden (Tilia Americana)', but in the Beech 

 this branch symmetry is preserved still longer, even until the 

 tree has attained a considerable height above the ground. 

 Nevertheless, as the tree continues to grow, it is subjected on 

 all sides to the influence of surrounding agents, especially 

 light, and the result is that the branches depart from their 

 original symmetrical position, and tend to arrange them- 

 selves concentrically so as to receive the greatest amount of its 

 influence. 



