106 TUB OAK. 



teen rays to the millimetre may be counted on the trans- 

 verse section of the wood. 



(2) The cambium cells situated between the rays 

 except when they suddenly commence to form a new 

 ray, as just described pass over into one or more of 

 the following elements of the wood proper viz., wood- 

 parenchyma, libriform fibers, tracheids, segments of the 

 vessels (see Fig. 28). 



When a cambium cell passes over into wood-paren- 

 chyma it first undergoes a few horizontal divisions trans- 

 verse to its long axis, and then we have a vertical row of 

 five or six parenchymatous cells, the walls of which do 

 not thicken much, but obtain small simple pits, and re- 

 tain part of their living contents protoplasm, nucleus, 

 starch-forming corpuscles, etc. and indeed present 

 much resemblance to the cells of the medullary rays 

 themselves. 



When the cambial cell becomes transformed into a 

 libriform fiber it does this simply by thickening its walls 

 at the expense of the living contents, etc., which soon 

 disappear. The cell undergoes no horizontal divisions, 

 and probably elongates very slightly. The thickened 

 walls become pitted with minute simple pits, and are 

 stratified and eventually lignified. 



In the case of the transformation of a cambial cell 

 into a tracheid everything is essentially as described in 

 the last paragraph, except that the diameter increases 

 and the thickening walls become marked with bordered 

 pits, quite similar to those of the pine, except that they 



