Biologic Forms of Erysiphem Cases of Non-Infection! 19 
now mostly disorganized ; here and there, however, a conidium 
could be seen which had formed a full-sized lobed haustorium, and 
in a few cases, a short weak hypha proceeded from the 
appressorium. 
On the 6th day the control wheat leaf (inoculated from the 
same source) was examined, and was found to be virulently infected, 
the inoculated area being now covered over with a continuous 
powdery Oidiutn- patch, and the epidermal cells of the leaf containing 
many hundreds of large lobed haustoria. In the barley leaf only 
numberless refractive bodies, the incipient or arrested haustoria, for 
the most part disorganized, could be seen ; in many cases they 
were still attached to the withered germ-tube of the conidium, but 
were often without any such attachment and quite solitary. 
(2) Several leaves, cut off from seedling plants of Bromus 
commutcitus, were inoculated with conidia either from B. mollis or 
from B. racemosus. Conidia from such a source had been previously 
found to be unable to cause any visible infection of B. commutatus 
(see 3). In the present experiments, also, no visible signs of infec¬ 
tion appeared on the leaves. Microscopic investigation, however, 
showed that the conidia had germinated and produced in many cases 
fair-sized haustoria, which had frequently developed the finger-like 
processes (Figs. 5 and 6). In a few of these cases, a short hypha 
was produced from the appressorium (Figs. 5 and 6). Nevertheless, 
no further growth beyond this stage of development was reached, 
and no true infection resulted. 
(3) Conidia were taken from oats, and sown on five wheat 
leaves (attached to vigorous young seedling plants); at the same 
time one leaf of one of these seedling wheat plants was inoculated 
with conidia taken from wheat. By the 5th day the leaf last referred 
to had become virulently infected, the epidermis at the inoculated 
place being nearly covered over with patches of mycelium, bearing 
hundreds of young conidiophores ; and nearly every epidermal cell 
contained one or more large lobed haustoria. No trace of external 
infection appeared on the five other wheat leaves, but on micro¬ 
scopic examination it was seen that many of the conidia had 
germinated and produced in the cells small refractive bodies— 
incipient or arrested haustoria—quite similar to those shown in 
Fig. 3. 
The results of the above experiments afford proof that the 
immunity to the attacks of the conidia of other “ biologic forms ” 
possessed by certain host-species is not due to the failure of the 
