Medical Officers of the Army of India.
1 1 5
surfaces. When the extremity of a cord reaches the base of a petiole, it is either
completely diverted outwards along its course, or divides, giving off a branch
towards the leaf, whilst the rest of it continues to ascend along the axis. When
the mycelium reaches the under surface of a leaf, the cord breaks up and ramifies
indefinitely, covering the epidermis with a coating of white filaments, and as this
advances the tissues of the lamina throughout become discoloured and die. Where,
on the other hand, it comes in contact with the upper surface, only a limited
amount of ramification occurs and growth is soon arrested without visible injury
to the leaf. It thus behaves in precisely the same fashion as the mycelium of
the common sclerotial blight of Ficus stipulata does.
The nutritional relations of the mycelium to the upper and lower surfaces
of the leaves are quite distinct. The upper surfaces are apparently related to
the mycelium just in the same fashion as the bark of the shoots is ; they merely
serve as a supporting surface over which mycelial filaments deriving their nutri
-
tion from other sites may travel, but the under ones are clearly a site for the
acquisition of large supplies of fresh nutritive material, as indicated by the
indefinite and rapid growth of mycelial elements occurring in connection with it.
But the great difference which exists between the upper and under surfaces of
leaves generally in their nutritional relations to parasitic or facultatively parasitic
fungi lies in the much greater facilities which the latter afford for the penetration
of mycelial filaments into the interior of the host-tissues, both from their gener
-
ally less resistant texture, and from the great excess of stomatic orifices which
they present. Taking this into account, together with the destructive effects
following the access of the mycelium to the inferior foliar surfaces, and the close
parallelism of the phenomena with those present in the case of the blight of
Ficus stipulata, there is little room to doubt that such penetration does occur.
At the same time it must be allowed that none of the specimens which I have
had the opportunity of examining have afforded any actual demonstration of the
fact. This, however, is not very astonishing, considering the exceptionally dense
texture of the leaves and the fact that in all the specimens the tissues had more
or less completely dried up ere they were subjected to examination—conditions
which are specially calculated to render the detection of intrusive mycelial ele
-
ments a matter of extreme difficulty. In the case of many uredinous blights, in
which the presence of Very large numbers of patches of fructification unequivocally
implies the existence of abundant and widely diffused mycelium within the
substance of the tissues, the demonstration of it is frequently a matter of very
great difficulty, even in perfectly fresh specimens, and where the host-tissues are
dense and the parasitic elements have been subjected to desiccation, the diffi
-
culty is naturally greatly increased.
As has been already pointed out, the phenomena attending the spread of
this disease are closely parallel to those occurring in the case of the sclerotial
Q 2