762 Blackman and Welsford . — The Development of the 
Polystigma rubrum is a fungus parasitic on Primus spinosa and 
P. insititia , and on the cultivated plum. Its attack is confined to the 
leaves, on which it produces orange-yellow sclerotial patches. These 
patches are each the result of a separate infection and they only spread 
to a limited extent ; with the fall of the leaf in autumn the host plant 
is freed from the disease. The life-history is almost identical with that 
of the leaf-scorch fungus ( Gnomonia erythrostoma ) of the cherry, but while 
the affected leaves of the cherry remain on the tree during the winter, the 
leaves attacked by P. rubrum fall at the usual time. This difference is no 
doubt related to the fact that in Polystigma there is a sclerotium, in which the 
perithecia develop as the leaves lie upon the ground, while in Gnomonia this 
structure is absent, so that if the leaves fell and rotted on the ground there 
would remain no basis for perithecial development. 
In this country P. rubrum does not seem at all abundant. Though 
o 
it is found in small quantities in various places the only locality known 
to us in which it occurs in any abundance is at Palling-on-Sea, Norfolk ; 
our attention was first drawn to the fungus in this locality by Mr. George 
Massee. 
Mycelium. The stages of actual infection of the leaf by a germ-tube 
from the secondary spore, which develops from the germinated ascospore, 
have been sufficiently investigated by Fisch ( 5 ) and Frank (6), and were 
not studied. The mycelium in its early stages shows thin-walled hyphae 
with cells containing a small number of nuclei, usually one to three. The 
hyphae push their way between the host cells (PI. LXX, Fig. 1) gradually, 
forcing them apart and bringing about their disorganization. In a mature 
sclerotial patch the host tissue is reduced to a few isolated cells with tannin- 
like contents which stain deeply ; the rest of the cells having been com- 
pletely absorbed (Fig. 2). 
The walls of the hyphae are at first thin (Fig. 1), but early in 
development the thin walls are modified into thick, gelatinous membranes 
which encroach on the cell cavity of the hyphae ; these membranes usually 
show fine pits (Fig. 3). The gelatinous wall is no doubt of the nature of 
reserve material, for it appears to be in part absorbed during the later develop- 
ment of the perithecium after the fall of the leaf. No storage of reserve 
material in the form of special cell contents was to be observed. 
In many well-developed sclerotial patches the hyphae form a continuous 
mass between the upper and the lower epidermis, the only remains of the 
mesophyll being the few scattered cells already mentioned. The hyphae 
generally contain in the fresh state an orange pigment which gives the 
blight colour to the sclerotial patches ; it is easily soluble in alcohol. 
It is to be noted that in the early stages of mycelial development the 
hyphae congregate, especially in the intercellular spaces beneath the 
stomata, and often push their way through the stomatal pore (Figs. 4 and 5). 
