PLATE XXII. 
Camera lucida drawing of transverse section of wood of yellow birch 
(Betula lutea) wood decayed by Polyporus pargamenus, X 800. The 
section, which was taken through two adjoining growth rings, shows 
the late wood on the left side and the early wood on the right side, 
with a group of vessels in each corner, and two pith-rays running 
horizontally through the center. The cells of the lower pith-ray have 
been completely dissolved out while the adjoining prosenchyma ele- 
ments of the summer wood remain unaffected. The layers of second- 
ary thickening of the elements in the early wood have been completely 
dissolved out, leaving only the middle lamella, a condition which 
gives the wood a skeletonized appearance. 
