176 Report of DEPARTMENT OF BACTERIOLOGY OF THE 
cellulars commonly occur and often slits or openings radiate 
from these for some distance. These result from mechanical 
‘Strains, doubtless caused by growth changes. 
Where such openings occur it.is usually evident that the 
middle lamella substance has split along its middle plane and 
a brightly refractive border line of this substance bounds each 
inner lamella externally along these intercellular slits and 
Spaces. In many cases, however, no such intercellular has de- 
veloped and the middle lamella substance extends as an appar- 
ently homogeneous layer of slightly varying thickness between 
the adjacent inner lamellae of the walls. At the junction of 
three or more cells this expands into an angular mass, com- 
pletely filling the space formed by. the meeting of their rounded 
contour lines. AS seen in section these ‘masses are most often 
triangular. There is some evidence from their deeper staining 
properties that these are denser than the thinner lamellar 
plates, and our observations on the rate of solution, to be dis. 
cussed later, are in accord with this idea. 
Various opinions have been held as to the composition and 
origin of the middle and inner lamellae of the walls. It has 
long been understood that parenchymatous walls of the kind 
under discussion are composed of cellulose. More recently it 
has been shown that “ cellulose” includes a group of closely 
related compounds. Moreover, if cellulose stains or solvents be 
applied to parenchymatous tissues it will appear that the inner 
lamellae are fundamentally cellulose, whereas the middle la- 
mella does not give the cellulose reactions. More critical ob- 
servations will show that the inner lamellae are rarely if ever 
homogenous but also contain substances other than cellulose. 
Cross and Bevan (1895:78, 89) in their discussion of cellu-_ 
loses make two groups, (1) the cellulose group, (2) the com- 
pound celluloses. They further sub-divide the cellulose group 
into three sub-groups: 
(a) Resistant to hydrolysis, e. g., cotton: 
(b) Less resistant to hydrolysis, found in grass stems, ete. 
(c) Low resistance to hydrolysis, found especially in fleshy 
roots and in seeds. , 
