Igor] THE MIDDLE LAMELLA 11 
as, the middle lamellae of the sclerenchyma. The green appear- 
ance of the sclerenchyma walls is probably not a differential col- 
oring, but is due to a combination of their original yellowish- 
brown with the blue stain. 
In sections treated with ruthenium red, the sclerenchyma and 
stone cell walls do not take up the stain to any appreciable 
extent. The walls of the fundamental parenchyma are clearly 
brought out, and are again plainly continuous with the middle 
lamellae of the sclerenchyma and stone cells. The indifference 
of the middle lamella in these thickened walls to methylene blue 
and ruthenium red, although this layer is directly continuous 
with the thin, readily-staining parenchyma wall, is in marked 
contrast to its character in the tissues of the spermatophytes 
Studied; it may be due to a change in the chemical composition 
of the middle lamella which occurred simultaneously with the 
deposition of secondary thickening layers. The fundamental 
parenchyma walls show no evidence of secondary thickening. 
They are stained by ruthenium red deeply and quite uniformly, 
Save that at the angles the wall is thicker, the triangular or quad- 
rangular area so formed is less deeply stained, and contains 
apparently a less dense substance than that forming the remain- 
er of the wall. This less deeply stained area corresponds to 
the thickened corners and the intercellular spaces of the scleren- 
chyma. The protoplasmic contents of the parenchyma cells are 
stained unevenly, the nuclei very deeply. 
In the vascular bundles, the parenchyma walls are deeply 
and uniformly stained by ruthenium red. These walls appear con- 
tinuous with the stained middle lamellae of the large vessels and 
Sieve tubes. In the walls of the vessels the middle lamella is 
relatively thin; the rest of the wall is uncolored. The middle 
lamella of the sieve tubes occupies a larger proportion of the 
wall thickness, the interior unstained layers being very thin. At 
corners formed by three or four cells the middle lamella appears 
more dense.and somewhat, though not greatly, enlarged. 
The staining reaction of the middle lamella in these elements 
is in contrast to its conduct in the sclerenchyma and stone cells, 
