The Biology of Polyporus Pargamenus Fries 93 
The individual cells of wood parenchyma are characterized 
by very thin walls (much thinner than those of the wood 
prosenchyma elements) and abundant simple pits, particu- 
larly on their end walls. On the other hand the wood prosen- 
chyma of yellow birch offers the usual features. It is char- 
acterized, however, by elements provided either with small 
simple pits, or more rarely, small but distinctly bordered pits. 
(Fig. 5.) In shape and size the bordered pitted elements 
resemble wood fibers rather than tracheids.*® 
yes 5: 
3ordered pit in vellow birch (Betula lutea) wood, transverse section, 
X, 3,000. 
Microchemical Reactions of Normal Wood.— The course 
of the destruction of the cell walls and the chemical changes 
occurring in them can be observed readily by studying tem- 
porary mounts of sections of decayed wood treated with the 
various microchemical reagents commonly employed for the 
recognition of cellulose and hgnin. Studies of this kind, 
* Many authors avoid distinguishing between such similar elements as 
wood fibers and tracheids by classifying both as wood prosenchyma. In 
many woods all gradations occur in the pitting of these elements from 
simple pits on the one hand to semi-bordered or bordered pits on the 
other hand. 
