176 STUDIES OF TREES 



stones and mortar as in Fig. 119, and finished with a layer 

 of cement at the .nouth of the orifice. This surface layer 

 of cement should not be brought out to the same plane 

 with the outer bark of the tree, but should rather recede 

 a little beyond the growing tissue (cambium layer) which 

 is situated immediately below the bark, Fig. 120. In this 

 way the growing tissue will be enabled to roll over the 

 cement and to cover the whole cavity if it be a small one, 

 or else to grow out sufficiently to overlap the filling and 

 hold it as a frame holds a picture. The cement is used in 

 mixture with sand in the proportion of one-third of cement 

 to two-thirds of sand. When dry, the outer layer of cement 

 should be covered with coal tar to prevent cracking. 



Trees that tend to split: Certain species of trees, like 

 the linden and elm, often tend to split, generally in the crotch 

 of several limbs and sometimes in a fissure along the trunk of 

 the tree. Midwinter is the period when this usually occurs 

 and timely action will save the tree. The remedy lies in 

 fastening together the various parts of the tree by means of 

 bolts or chains. 



A very injurious method of accomplishing this end is 

 frequently resorted to, where each of the branches is bound 

 by an iron band and the bands are then joined by a bar. 

 The branches eventually outgrow the diameter of the bands, 

 causing the latter to cut through the bark of the limbs and 

 to destroy them. 



Another method of bracing limbs together consists in 

 running a single bolt through them and fastening each end 

 of the bolt with a washer and nut. This method is prefer- 

 able to the first because it allows for the growth of the limbs 

 in thickness. 



A still better method, however, consists in using a bar 

 composed of three parts as shown in Fig. 121. Each of the 



