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, &ce.—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 
