156 College of Forestry 
position. In some the tertiary layer no longer gives the 
characteristic color reactions when treated with various 
lignin reagents, but remain colorless. In other cells the 
secondary lamella of the cell-wall also remains colorless and 
undoubtedly consists of practiéally pure cellulose. As a 
general rule both the secondary and tertiary lamelle remain 
in place until after all the cell-wall but the middle lamella 
becomes delignified. After all the layers of the cell-wall but 
the primary lamella have become delignified the destruction 
of the cellulose commences. Those layers of the cellulose 
walls which remained colorless after treatment with lignin 
reagents take on a pronounced violet color when treated with 
cellulose reagents, such as chlorzinc-iodine. Groups of these 
cells sometimes are seen in which the middle lamelle are 
colored brown with chlorzinc-iodine, while the secondary and 
tertiary lamelle are colored a deep violet and frequently 
exhibit a pronounced swelling. Occasionally these cellulose 
layers swell up to such an extent that the cell lumen is 
scarcely visible. In general the dissolution of the cellulose 
does not occur within any one cell until some time after its 
wall, excepting the middle lamella, has become completely 
delignified. Within the individual cell the cell corners, 
where the middle lamellze often are thickened characteristi- 
cally, respond the longest to tests for hgnin. Owing to the 
great degree of lignification exhibited by the middle lamella 
it remains in place long after both the tertiary and secondary 
layers of the cell-wall have disappeared. After the secondary 
layers of thickening have been dissolved the resulting woody 
tissue has a skeletonized appearance. It has all the elements 
but their walls consist principally of the middle lamella, the 
other lamelle having practically all been dissolved away. 
These middle lamellz retain the nature of lignified walls and 
offer considerable resistance to further dissolution. They 
are the last part of the cell-wall to disappear and, as stated 
before, remain in place long after the other lamellze have 
completely dissolved. _ However, they too gradually grow 
thinner and thinner until they break up into irregular frag- 
ments. It is not until the middle lamelle have disintegrated 
