Medical Officers of the Army of India.
39
a very limited extent in initiating the deeper affection. The leaves in some cases
show an abundance of superficial primary discs like those from which the com-
mon secondary discs originate. A certain number of secondary subcuticular
discs may also occasionally be detected, but these are ordinarily of very small
size, as the majority soon penetrate deeper to give origin to the characteristic
patches. When they do so they remain for a time recognisable in relation to the
patches, but soon dry up and entirely disappear.
In determining the distribution of the algal elements throughout the affected
areas, the principal difficulty lies in the fact that the dead host tissues are so
darkly coloured that, unless sections are very thin, they are opaque, and so
brittle that thin sections are extremely liable to break up into fragments. The
results obtained from the examination of a very large number of sections were as
follows:—Beneath the epidermis over the general surface of the upper side or
the affected area, there is a fairly evenly distributed layer of algal cells which
separates the epidermis from the subjacent stratum of palisade cells (Plate I,
fig. 8). There is as a rule no such general thalloid algal stratum beneath the
inferior epidermis, but merely scattered masses of algal cells penetrating to a
considerable depth into the substance of the lamina and giving off emergent
filaments, which rupture the epidermis and appear on the surface (Plate I, fig. 9).
The thalloid stratum beneath the superior epidermis is very thin over the greater
part of its extent, consisting of one or two strata of cells only, but here and
there it increases in thickness, and at these points tufts of emergent fertile
filaments arise. In some cases the algal cells are of green colour, but in many
cases, especially in the inferior masses, there is not a trace of green present and
the contents are of a brilliant yellow. The distribution of the superficial masses
of algal elements, the conspicuous changes in the tissue of the lamina throughout
its entire thickness, and the presence of emergent fertile filaments on both
surfaces, clearly show that the alga must penetrate the leaf throughout, but it is
very difficult to obtain direct evidence of the fact, and it was only after preparing
a very large number of sections that specimens were obtained satisfactorily
demonstrating it.
The tissue of the leaves is extremely dense and the thick walls of the cells
acquire a very deep brown colour in the central portions of the affected areas, the
contents at the same time shrinking and becoming reduced to brown amorphous
masses and particles. In the peripheral parts, corresponding with the yellow
superficial areolæ various stages of advance towards the conditions present in
the centre are visible. The cell-walls are yellowish and their contents more or
less modified, the chlorophyll gradually disappearing, and the protoplasm
becoming lumpy and granular. In many places, even in the thoroughly altered
central areas, peculiar circular, brilliant green bodies are present (Plate I, fig. 4)
in considerable numbers among the dead and discoloured tissue elements, but
whether these are really algal cells or mere products of the alteration in the host