202 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



colour which becomes toned down to various shades 

 of olive, gray, brown, &c., as the layers of cork increase 

 with the age of the part. It is because the corky 

 layers are becoming thicker that the twig passes from 

 green to gray or brown as it grows older. Now these 

 green living cells of the cortex are very important for 

 our purpose, because, since they contain much food- 

 material and soft juicy contents of just the kind to 

 nourish a parasitic fungus, we shall find that, whenever 

 they are exposed by injury, &c., they constitute an 

 important place of weakness nay, more, various fungi 

 are adapted in most peculiar ways to get at them. 

 Since these cells are for the most part living, and 

 capable of dividing, also, we have to consider the 

 part they play in increasing the extent of the cortex. 

 (3) The third of the partly natural, partly arbitrary 

 portions into which we are dividing the cortical jacket 

 is found between the green, succulent cells (pa) of the 

 cortex proper (which we have just been considering), 

 and the proper cambium, Ca, and it may be regarded 

 as entirely formed directly from the cambium-cells. 

 These latter, developed in smaller numbers on the out- 

 side, towards the cortex, than on the inside, towards 

 the wood, undergo somewhat similar changes in shape 

 to those which go to add to the wood, but they show 

 the important differences that their walls remain un- 



