14 BOTANICAL GAZETTE (JULY 
lamella. Intercellular spaces occur in the xylem where three or 
four cells abut; they appear as triangular or quadrangular breaks 
in the uniformly stained middle lamella. 
Comparing the results obtained from methylene blue and 
ruthenium red, we find that by both the whole xylem wall is 
stained, the middle lamella being differentiated by the red and 
not by the blue. Since methylene blue stains nitrogenous sub- 
stances equally well with pectic acid, and does not stain cellulose 
or callose, while ruthenium red stains pectic derivatives and not 
nitrogenous substances, it appears that the secondary layers of 
the xylem, as well as the thin layer of the cambium wall which 
does not take up ruthenium red, contain a mixture of nitro- 
genous and pectic compounds. The use of orseille, a cellulose 
stain, gave no satisfactory results, showing that celulose, if pres- 
ent at all in these walls, is of minor importance. The staining 
power of the tori of the pits indicates that they represent in the 
main the same layer as the middle lamella; this does not, how- 
ever, preclude the possibility of their also possessing a thin, 
subsequently deposited layer. The deeply stained layer lining 
the pits is an example, of which others will be mentioned later, 
of the possible deposition of a pectic layer very late in the his- 
tory of the growth of the cell wall. 
The relations of the middle lamella, tori, intercellular spaces, 
and thickening layers appear about the same in older pine wood 
stained with ruthenium red as in the young tissues already 
described. 
In cross sections previously treated with acid alcohol and 
exposed for one hour to fuchsin, which belongs to the same class 
as methylene blue, the xylem walls are stained throughout. 
Other tissues, except the contents of the cambium and collen- 
chyma cells, are unstained. 
Cross sections of older pine wood were left in ponceau solu- 
tion for three or four days. The xylem is quite uniformly stained 
vermillion, the middle lamella generally appearing darker than 
the inner layers of the wall, but the distinction of coloration is 
not strongly marked, and the difference may be wholly due to 
the difference in density. 
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