1879. ] Microscopical Fungi Infesting our Cereals. 619 
of Erysiphe, and so I understand under. the name of “ mildew T 
the mycelium of a parasitic fungus which, sometimes very thin 
and cobweb-like, sometimes in a thick, felty layer, disturbs the 
development of the infected plant, particularly by ọbstructing the 
stomata, perhaps also by drawing its nourishment from out of the 
infected organs by means of little wart-like processes, through 
which the mycelium is attached to the plant on which it feeds. 
Among the great many varieties of Exysiphe, which are to be 
found on the leaves of different trees as well as on a great many ` 
ornamental and useful plants, we have to deal here with one 
species, Æ. graminis, which in certain years is to be found in 
abundance, covering the leaves of the several cereals. 
The real nature of this parasite has likewise but recently been 
discovered, and it requires a good microscope to discern the 
minute organs of fructification. 
Soon after the formation of the “ mildew ” has commenced, we 
see some of the mycelium-threads growing upwards in a vertical 
direction. These threads become segmented into several cells ; 
each of these oval cells is separated in the same way as we have 
seen in Sphacelia, is transformed into a conidium, and by means 
of these conidia, the mycelium is propagated during the summer 
months, with incredible rapidity under favorable conditions. 
Towards the close of the period of the growth of vegetation 
we see in a very interesting way that peridia are formed by means 
of a sexual process. ; 
At the crossing point of two mycelium-threads two cells of a 
somewhat unlike shape are differentiated. The larger one, issuing 
from the inferior hypha, is club-shaped. It is (according to De 
Bary, to whom we are indebted for very careful investigations 
of this subject) the female “ascogonium,” as he terms it, against 
which thesmuch thinner “ pollinarium” (growing up from the 
inferior hypha) leans. closely. The contents of the pollinarium 
being absorbed by the female organ, the latter begins to swell or 
enlarge as the pollinarium disappears. In the mean time a num- 
ber of secondary threads are developed at the basis of the asco- 
gonium. These threads surround and enclose the ascogonium, 
' producing numerous lateral as well as inward growing ramifica- 
tions. The lateral threads weave themselves into a compact tis- 
sue which afterwards hardens, becomes brownish and forms the 
outer coat of a peridium, while the space between this coat and 
VOL. XIII.-—-NO. X. 
