122 Potter. — On the Occurrence of Cellulose in the 
thickening layers of the woody elements, which, on treatment with colour 
stains, gave indications of the presence of cellulose, suggesting that the 
action of the Fungus was one of delignification. Biffen also observed 
a sharply defined zone of cellulose surrounding many of the pits, especially 
in the vessels. He considers that the action of Bulgaria polymorpha upon 
Oak-wood causes the dissolution and probably decomposition of the lignin, 
and has assumed the existence of a delignifying enzyme to explain these 
results. 
Czapek (’99) claims to have isolated a delignifying enzyme from 
Merulius lacrymans and Pleurotus pulmojiarius. From the fungus-extract, 
by precipitation with alcohol, he obtained a white precipitate, soluble in 
water, which destroyed the lignin of the woody cell-walls. This fungus- 
extract, he states, lost its wood-destroying properties when once boiled. 
Hermann von Schrenk (’00) has described a disease of the Red Cedar 
caused b y Polyporus Juniperinus^ which attacks the walls of the wood-fibres, 
and extracting the lignin substances leaves a basis of almost pure cellulose. 
The amount of wood-fibre reduced to cellulose is very considerable, and 
it is supposed that some very potent lignin-splitting enzyme is concerned 
in the changes which are brought about. Schrenk (’03) also described 
a delignification in the wood of Fraxinus due to Polyporus fraxin oph ilus , 
which is said to give off an enzyme which attacks the inner parts of the 
wood-cells, extracting the lignin, and leaving an impure cellulose. 
Some evidence may be gathered with reference to cellulose occurring 
normally in the xylem. 
Writing in 1 86 o Sanio (’60) states that the tertiary thickening gelatin- 
ous layer of lignified cells was first pointed out by T. Hartig and confirmed 
by von Mohl, and finally claimed by Schacht as an element present in all 
woody cells, which never becomes lignified but consists always of pure 
cellulose. Sanio regards this latter statement as too sweeping, and maintains 
that the share which this tertiary layer takes in the thickening of the woody 
cells varies greatly, sometimes being very strongly pronounced, at other 
times hardly perceptible. He mentions Acer platanoides as a stem in 
which this layer, which is coloured violet with chlor-zinc-iodine, is most 
distinctly to be observed ; and adds a list of seventeen species in which this 
cellulose thickening is found in the wood-fibres. 
In his general considerations on the structure of the Oak (p. 94 ), Hartig, 
speaking of the wood-fibres, says that in the walls of these fibres two, often 
three, layers can be distinguished, differing in their chemical behaviour. 
The most external, always a very thin layer, which is united with the 
corresponding layer of adjacent organs, encloses a very thick inner layer. 
This is lignified, and only in isolated cases is bounded internally by a third 
layer, which colours blue with chlor-zinc-iodine. Hartig himself makes 
only slight further allusion to this occurrence, as when discussing Hydnum 
