1901] THE MIDDLE LAMELLA 15 
The xylem walls of young wood are not stained after a half- 
hour’s treatment with acidulated nigrosin solution. This fact, 
by itself, would mean the absence of nitrogenous substances ; 
but, as against that furnished by methylene blue, and by ponceau, 
which belongs to the same class as nigrosin, such negative evi- 
dence is of little value. 
In longitudinal sections of young tissues treated with acid 
alcohol and stained with ruthenium red, the tori of the pits are 
stained deep red. The appearance of the tori and of the red- 
staining bars between the pits, which bars are referred by Dippel 
(2) to the remains of the Zzischensubstanz, is well shown by him 
in his figure 397. In many cases, probably in partly developed 
pits, the stain is only a deep spot at the center. In some cases, 
an unstained center is surrounded by a stained ring. The pits 
are not so noticeable where they connect with the cells of the 
medullary rays, but where the ring appearance just described is 
found in older wood, the tori of pits so situated are completely 
stained. 
Both the walls and the cell contents, especially the nuclei, 
of the medullary rays are deeply stained by methylene blue, both 
in cross sections previously treated with acid alcohol and in those 
not so treated. This is true of both young and old wood. 
Similar results are effected by ruthenium red. In cross sections 
of old wood stained with ruthenium red and then exposed for a 
few seconds to methylene blue, the red stain of the xylem is 
replaced by the blue, while, if the action of the blue has not been 
too long continued, the medullary rays remain red. The greater 
affinity of the xylem walls for the blue than for the red is due 
probably to their containing nitrogenous and pectic compounds, 
both of which take up methylene blue, while the. former, unless 
Specially treated, have no affinity for ruthenium red. 
N cross sections treated for one half hour with acidulated 
nigrosin solution, the medullary rays and their cell-contents take 
up the stain freely. 
In cross sections of older pine wood treated for three or four 
days with ponceau, the medullary rays are unstained. 
