176 
MR. H. MARSHALL WARD ON THE STRUCTURE 
I now pass on to a more detailed description of the white “ disease-spots” on the 
leaves (Plate 10, fig. 1). On its first appearance the spot is pale and greenish, and not 
sharply marked off from the surrounding tissue, and it requires close watching to be 
sure when it first becomes visible to the unaided eye ; shortly afterwards, the central 
parts of the enlarging fleck are pure white, resembling powdered chalk, especially w'hen 
the air is warm and still, and the conidia to be referred to have accumulated in large 
quantities. As the white speck ages and enlarges centrifugally, it turns more ash- 
coloured or yellowish in the centre, and finally becomes brownish, or even dark brown, 
and the patch of tissue is dead. 
These spots appear on both sides of the leaves, and are alike, except that the white 
stands out more sharply in contrast with the darker glossy green of the upper side of 
the leaf. On a warm, still morning, it is possible to collect relatively large quantities 
of the white chalky powder (conidia) from the more active flecks, and it will be seen 
how important this must be in the reproduction and spread of the fungus causing the 
infection. 
The white flecks are confined to the mesophyll of the leaf, and can be seen sharply 
bounded by the vascular bundles of the venation—for instance, in the fork "whence 
two chief veins diverge—whereas they fade imperceptibly into the green of the 
mesophyll. The leaf is not thickened at the infected spots, but it is very noticeable, 
as the spot increases in age, that it becomes thinner and dries up or rots ; in the 
former case cracking and tearing away from the healthy tissue, and, in the latter, 
falling down as putrid shreds (fig. 1). The difference depends on weather. In both 
cases the bits may soon disappear, and the leaf look as if a piece had been nibbled 
out. 
It will thus be seen that the diseased condition is confined to a given area ; the 
spots are local centres, and do not spread indefinitely over the leaf. I have counted 
57 on a leaf less than inches broad and long, and many more can co-exist on that 
area. In some cases spots run together as they age. As I shall show later, each 
spot spreads from one centre only, i.e., from a stoma through which a germmal tube 
from a spore has passed ; the stomata are on both sides of the leaf. 
A curious, though by no means isolated, phenomenon is presented in the case of old 
leaves, which have shown the spots at a late stage of their life, and then turned 
yellow before the spots reach their matured condition : this is the existence of a vivid 
green ring around the spot, and is, without doubt, due to the mycelium of the fungus 
keeping the cells active after their neighbours are dead. I have noticed the same fact- 
in the case of other parasitic fungi. 
Sections through a young white spot show that a very delicate, copiously branched, 
mycelium exists between the cells of the mesophyll, both in the palissade and spongy 
tissue (figs. 7-14). Closer examination shows that the mycelium is segmented at 
rather long intervals, but the septa are very difficult to observe without reagents, 
owing to their thinness and that of the outer walls, and to the dense, finely granular 
