GROWTH IN THICKNESS 



79 



summer layers, and which the softer spring or wet 



weather growths. 



This growth in thickness by means of the cambium 



layer takes place only in dicotyledons. Monocotyledons 



do not increase at all 

 (except in one or two 

 genera and then by a 

 different method) for 

 they have no cambium, 

 and in consequence 

 also no bark. Com- 

 pare, for instance, a 

 Palm with a Rain-tree 

 (PITHECOLOBIUM 

 SAM AN), or Indian 

 Cork (MILLING- 

 TONIA), or any ordi- 

 nary tree. The latter 

 have a bark that can 

 be easily stripped off 

 the younger branches, 

 but there is none 

 on the Palm, its 

 substance is of the 

 same nature right 

 through, though harder 

 towards the circum- 

 To show how such a tree grows in f ere nce being merely 



thickness at or near the surface, not by j ,1 , . , 



. , y covered on the outside 



expansion from the middle. 



_, . , by the bases of the 



The wire has not bitten into the tree, 



but the tree has grown round the wire. leaves th at have 



fallen. 



FIG. 16 



TRUNK OF A TREE, ACACIA 



MELANOXYLON, WHICH HAS BEEN 



USED AS A POST FOR TYING THE 



WIRE OF A RAILING 



