106 THE OAK 



rays to the millimetre may be counted on the transverse 

 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 fibres, 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 

 transverse 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 retain part of their living contents proto- 

 plasm, nucleus, starch-forming corpuscles, &c. and 

 indeed present much resemblance to the cells of the 

 medullary rays themselves. 



When the cambial cell becomes transformed into a 

 libriform fibre it does this simply by thickening its 

 walls at the expense of the living contents, &c., which 

 soon disappear. The cell undergoes no horizontal divi- 

 sions, 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 dia- 

 meter increases and the thickening walls become 

 marked with bordered pits, quite similar to those of the 



