NoTES ON THE ERYSIPHACEAE 15 
while other trees, growing intermixed with the Berderis and 
Corylus (such as Carpinus, Betula, Fagus, Franinus, etc.), 
which in other districts are liable to be attacked, here remained 
perfectly free from the Phyllactinia, was led to the conclusion 
“dass Phyllactinia suffulta eine Sammelspecies darstellt, and dass 
die auf den verschiedenen Nahrpflanzen beobachteten Formen, 
wenn auch vielleicht nicht eine jede einer besonderen, doch min- 
destens mehreren verschiedenen, morphologische einander sehr 
ahnlichen oder gleichen Arten angehoren, welche nur auf ganz 
bestimmten Nahrpflanzen gedeihen. Zweifelsohne liegt bei PAy/- 
factinta und wohl auch noch anderen Gattungen der Mehlthau- 
pilze beziiglich der Speciesfrage dieselbe Erscheinung vor, wie sie 
bei den Rostpilzen aufgedéckt worden ist und als Spectalisrung 
des Parasitmus bezeichnet wird.” 
Reviewing the experiments given above, I should like to point 
out with reference to those, (1), (2) and (3), in which conidia were 
taken from one host-plant and sown on another, that such experi- 
ments require the greatest precautions being taken to ensure that 
the plant on which the conidia are sown has not been naturally in- 
fected previous to the experiment, and that it is not liable to infec- 
tion during the course of the experiment. Two cases which show 
the necessity of the greatest care being taken in this direction have 
come under my notice. A plant of Czzeraria, taken direct from 
a greenhouse in which the plants were apparently free from mildew, 
was placed in a greenhouse where the cinerarias were badly af- 
fected with Ozdium, and kept covered over by a bell-jar. Ex- 
periments were made with the object of trying to infect this 
Cineraria with an Oidium occurring in the same greenhouse on 
Celsta, and for this purpose the bell-jar was removed two or three 
times to allow the placing of the Ozdium-conidia on the leaf, and 
subsequently to observe if infection had taken place. The experi- 
ment failed, as on the marked places on the Crweraria-leaf where 
the conidia of the Ce/sia-Oidium were sown, no Ozdium appeared. 
In other spots on one or two of the leaves experimented with, 
however, an Ozdium began to grow, and spread gradually in large 
patches over the leaf, producing conidiophores in large numbers. 
A microscopic examination of the conidia of this Ozdium showed 
that it resembled the Ozdium that grew on the cinerarias in the 
