182 



A TEXTBOOK OF BOTANY [Ch. IV, 10 



Vertical stems are radially symmetrical, or alike all around, 

 both externally and internally. When horizontal or oblique, 

 however, as with branches, they are often excentric, having a 

 greater thickness of wood on the lower side, in obvious ad- 

 justment to the support of the branch against the leverage 

 imposed by its increasing length and weight. In such cases, 



also, there is frequently a thick- 

 ening in the lower angle be- 

 tween branch and stem, on 

 the principle of a bracket, 

 which is present in even 

 greater development at a place 

 of much greater strain, viz. 

 at the angle where a verti- 

 cal trunk joins spreading 

 roots. Here the bracket often 

 amounts to a buttress, as well 

 shown in old Elms, and even 

 more strikingly in some trees 

 which grow in the tropics. 

 The principle of resistance to 

 strain explains also the form 

 of the main branches in the 

 deliquescent type, for com- 

 monly they rise almost vertically from the trunk, and turn 

 gradually outward, becoming vertical again at the foliage- 

 bearing tip. 



Stems have not only to support the great mass of the foli- 

 age and also their own considerable weight, but must like- 

 wise resist lateral pressure from winds, which exert great 

 power against the foliage and therefore strong leverage on 

 the stems. Corresponding thereto is the tough-elastic 

 texture of the stems, whereby they are enabled to yield to 

 winds in a manner to shed off their force, as one can see in 

 any great trees in a storm. Where strong winds prevail in 

 one direction during the season of growth, a tree may be held 



Fig. 124. — The symmetry of a 

 lawn tree, the weeping Birch. 

 (From Bailey.) 



