CH. IV, 13] 



ECONOMICS OF STEMS 



211 



with a part of a bud of the stock, in which case the resultant 

 bud has the tissues of the two parents intermingled in diverse 

 ways. Such chimaeras, accidentally produced, are not un- 

 common in Oranges, or even in Apples, which sometimes have 

 one segment of skin differing sharply in color or texture from 

 the remainder. 



An important economic aspect of stem structure is in- 

 volved in the new practice of tree surgery. In order to pre- 

 serve valuable trees, it is now customary not only to prune 

 away branches seriously affected by disease, 

 but also to clean out cavities thus caused, 

 and fill them with cement, in imitation of 

 the methods successfully practiced by 

 dentists with teeth. Experience, however, 

 is hardly justifying earlier expectations, for 

 such cement-filled cavities, though seem- 

 ingly at first satisfactory, often decay next 

 the cement, which shrinks slightly in setting 

 and allows sap to exude and Fungi to enter, to strengthen a 

 Besides, the rigidity of the cement fits ^f 'SStet*! 

 badly with the elasticity of trees which in many cases, is 

 must sway in the wind, while its weight in . between t wo Iboits 

 some positions is a serious strain upon thin instead of the single 

 cylinders of wood. A promising, though 

 rather expensive substitute, is a filling of 

 wooden blocks set in an elastic, antiseptic material like tar. 

 In other details tree surgery has made real progress, e.g. in 

 the supporting of weak branches by chains and bolts, the 

 former of which permit a free motion in the tree, while the 

 latter prevents that choking of the bark which follows the 

 use of encircling bands (Fig. 155). The subject is still in 

 the developmental stage, on which account it offers a tempt- 

 ing field to incompetent practitioners, and even impostors, 

 against which type of " tree-surgeons " the owner of trees 

 must be upon guard. 





