NoTES ON THE ErRYSIPHACEAE 13 
rigkeiten, die Frage der Wirthzugehorigkeit fiir eine grosse Anzahl 
von Arten durch experimentelle Untersuchungen klargelegt worden 
ist.’’ Further, with reference to the fact that in my monograph a 
great reduction of species was made, Neger remarks : “ Inwieweit 
dieses summarische Verfahren berechtigt ist, muss die Zukunft 
lehren. Hieraus ergibt sich aber das zweite, einer Losung dring- 
end bedirftige Problem: Welcher der bisher auf gestellten, auf 
morphologische Merkmale begriindeten Erysipheenarten erweisen 
sich physiologisch als solche ? oder um die von Rostrup bei den 
Uredineen angewandte Bezeichnung zu gebrauchen: Werden 
nicht viele der bisher als morphologisch gleich erkannten Formen 
in ‘biologische’ Arten aufzuldsen sein ?”’ 
The question of the possibility of the occurrence of “ biolog- 
_ical species”’ in the Erysiphaceae is one of primary importance, 
not only from the scientific side, but also from the economic point 
of view—for if “biological species” do exist, then we should 
endeavor to ascertain if the species of mildew which cause such 
widespread damage to cultivated plants belong to this class. If, 
for instance, it were found that the ‘‘ hop-mildew”’ constituted a 
‘biological species,’ then we should know from which of the 
common weeds of the hop garden (Spiraea, Ulmaria, Potentilla 
spp., Epilobium spp., etc.) which serve as the host-plants of the 
morphological species called Sphacrotheca humuli, it could pass 
on to the hop; or whether, as might possibly prove to be the 
case, the spread of this hop-disease is dependent on infection from 
hop-plant to hop-plant. 
It will be well here to review the evidence with regard to this 
point. Direct experiments, and observations bearing on the sub- 
ject are, so far as Iam aware, very few in number. The experi- 
ment of taking the conidia of a mildew growing on a certain host- 
plant, and sowing them on the leaves of another plant has been 
performed by three botanists : (1) Magnus (in Bericht. deutsch. 
bot. Gesellsch. 16: 68. 1898) has stated that the conidia of 
Sphaerotheca humuli (DC.) growing on the hop will germinate 
and produce a conidia-bearing mycelium when sown on the 
leaves of Taraxacum officinale. (2) Iwanowsky (32) made some 
experiments in connection with the Ozdium* occurring on the to- 
*This Oidium De Thiimen described as O. fadaci; it is in all probability the 
conidial condition of Erysiphe cichoracearum DC. (see monograph, p. 294). 
