Igor | THE MIDDLE LAMELLA 13 
brown and fuchsin is worthy of note; both these stains belong 
to the class in which a basic coloring matter is united to an acid; 
according to Mangin, all the stains of this class stain pectic 
compounds and do not color cellulose or callose. 
In unstained cross sections through the xylem, the middle 
lamella is to be distinguished from the rest of the wall by its 
different refractive power, at some levels of focusing appearing 
brighter, at other levels darker, than adjacent layers. The same 
is the case in cross sections through old pine wood, except that 
here the middle lamella appears, if anything, more dense than 
in the young wood. 
In cross sections through young wood treated with acid 
alcohol and methylene blue, the whole xylem is colored green. 
The middle lamella is distinguishable only by its greater density, 
as also in sections not previously treated with acid alcohol. 
Xylem walls in cross sections treated with acid alcohol stain 
more deeply with ruthenium red than in sections not so treated. 
The middle lamella stains much more deeply than the inner 
layers, the boundary between the lightly and deeply stained por- 
tions being sharply marked. The difference in depth of stain 
is much more apparent in acid alcohol sections than in the 
others. The tori of the pits in acid alcohol sections are stained 
deep red; this occasionally occurs also in sections not treated 
with acid alcohol. In some cases in acid alcohol sections, the 
torus appears plainly to be a continuation of the middle lamella. 
In some cases also the middle lamella appears continuous with 
the interior layer of the pit cavity, which stains deeply. The 
middle lamella is more plainly differentiated in xylem of the 
present year’s growth. In some preparations the oldest cells of 
the present year’s growth (the spring wood) have the whole 
wall quite deeply stained. These facts may indicate that it is 
in the younger xylem walls that pectic substances exist in 
purest form. Where splitting of the walls occurs in preparing 
the section, it is sometimes on the line of demarcation between 
the middle lamella and the less stained layers; very often, how- 
ever, the split is approximately through the center of the middle 
