4 Biologic Forms ' of the Erysiphaceae. 1 29 
a normal uninjured leaf of barley and of wheat. In the eleven cases 
in which the barley leaves, on which the conidia were produced, had been 
treated previously with alcohol, in the four cases in which heat had been 
used, and in the one case in which ether had been used, the conidia proved 
totally unable to cause any infection on the barley leaf, while producing in 
every case full, and usually virulent, infection on the wheat leaf. In 
a single case 1 i only, the conidia produced on a barley leaf previously treated 
with alcohol, when sown simultaneously on a normal uninjured leaf of 
barley and of wheat, gave rise on the barley leaf to a small patch of 
mycelium bearing a few conidiophores. The Fungus, at an early stage, 
however, showed evident signs of being unable to develop fully, and soon 
died away. On the wheat leaf full infection resulted, and the Fungus 
persisted until the death of the leaf. 
In order to see, in these cases in which the conidia produced on the 
treated barley leaves infected wheat leaves, whether the infection-powers of 
the Fungus would vary in subsequent generations, conidia of the successive 
generations produced on wheat, up to the sixteenth generation, were 
cultivated. In these experiments 21 1 leaves of barley and of wheat 
were inoculated, and the conidia infected fully, and in most cases virulently, 
the wheat leaves, while never producing any sign of infection on the barley 
leaves. 
These experiments demonstrate the fact that the infection-powers 
of a ‘ biologic form * are not altered by its residence for one generation on 
a strange host-plant treated in the manner described, a fact which gives 
some evidence in favour of the idea of the hereditary nature of the infection- 
powers of some ‘ biologic forms.’ The results of other experiments which 
I have recorded (2) show, however, that the infection-powers of some 
‘ biologic forms ’ may be considerably influenced by the effect of a new 
host-plant 2 . 
The results obtained in the present series of experiments may thus be 
summarized : — 
(1) Susceptibility can be induced not only by various kinds of 
mechanical injury, but also by such interference with the normal functions 
of the cell as follows the application of anaesthetics and heat. 
(2) The conidia of the first generation produced on leaves of a strange 
host-plant previously subjected to the action of alcohol, ether, or heat retain 
the power of infecting their original host, and do not acquire the power of 
infecting normal leaves of their temporary host. 
1 See page 145, Exper. No. a 99. 
2 Thus, the conidia of the form of E. Graminis growing on Bromus racemosus are not able to 
infect B. commuiatus. When this form, however, is sown on B. hordeaceus , the conidia of the first 
generation prove able to infect B. commutatus . Normal (untreated) leaves of the host-species were 
used in these experiments. Presumably, the change of infection-powers in this case is connected 
with the change of nutrition. 
K 
