260 The Living Plant 



and horizontal positions permit them to yield greatly without 

 damage, and thereby even to shed their burdens (figures 14, 15). 

 No doubt the protective adaptation involved in the conical shape 

 has operated along with the photosynthetic considerations 

 earlier mentioned (page 56) to fix this form for evergreen trees, 

 which in general are commonest in the snowiest regions; while, 

 correlatively, the danger involved in the accumulation of snow 

 upon the leaves borne by upwardly springing branches, like those 

 of most of our deciduous trees, is doubtless one factor in making 

 such trees drop their leaves in the winter. 



This mention of the shapes of trees makes this a suitable place 

 to consider their modes of resistance to certain other strains. 

 The stems of trees have not only to carry great masses of foliage 

 high up in the air, but also to support it out laterally for con- 

 siderable distances, and all in opposition to a heavy downward 

 strain from gravitation. In some trees, conspicuously those of 

 the cone-shaped evergreen type (figures 14, 15), the branches 

 spread horizontally from a central upright trunk; but this ar- 

 rangement, however advantageous from other points of view, 

 is mechanically the worst for resistance to gravitational strains, 

 and is only possible with comparatively slender branches and 

 special methods of strengthening the same. Thus, bracket-like 

 swellings often occur in the angles between such branches and 

 stems, while extra material is commonly placed all along the 

 under side of the branch, making it excentric in cross section. In 

 such cases the extra material acts much like a long stiff spring 

 bent upward just enough to counterbalance the weight of the 

 branch, whose horizontal position is maintained by the counter- 

 action of the two forces, as is shown quite conclusively by the 

 very great bending of such branches when spring and weight are 

 allowed to act together by the inversion of the tree (figure 90). 

 But a cone-shape of trees is uncommon in comparison with that 

 in which great branches, often well-nigh as large as the trunk, rise 

 up therefrom at sharp angles, swing gradually outward to near 



