89 
their appearance at the same time and are consequently exposed 
to the same external conditions. Generally the barleys suffer 
from mildew one season, and rust another, but we have not 
sufficient data so far to forecast which will be the more serious 
pest. Eriksson, to explain certain facts in connection with 
epidemics of yellow rust has been led to formulate his much- 
discussed “ mycoplasm” hypothesis. The essential points of 
this are that the wheat grains themselves contain within some of 
their cells a portion of the protoplasm of the yellow rust. This. 
fungus protoplasm, or mycoplasm, lives for a time symbiotically 
with its host, but under certain external conditions it is sup- 
posed to be capable of further development, resulting in the 
production of the mycelium and finally of the spores of the 
rust. This conception of the handing on directly from the 
maternal parent to the offspring of the actual cause of disease 
has not met with general acceptance. It runs counter to al] 
ones knowledge of heredity and one may now dismiss the 
subject, for it has been shown that Eriksson when attempting 
to demonstrate the actual mycoplasm and its development was 
misled, largely by faulty technique. 
We may say definitely that we know of no well authenticated 
case where disease can be transmitted in this way, but we do 
know definitely that the tendency or the liability to disease is 
inherited, and, that given suitable conditions, the plants will 
certainly become diseased. Many examples might be quoted 
but I must content myself with a few. If we take four barleys, 
Hordeum spontaneum, Hl. violaceum, H. japonicum, H. 
hexastico-furcatum, and keep them under observation season 
by season, the first will be found practically immune to the 
attacks of Erysiphe graminis, whilst the remaining three will 
show gradually increasing degrees of virulence in the attacks 
of the parasite in the order quoted. In the same way wheats 
such as Engrain commun are always practically rust free, whilst 
Michigan Bronze is badly infected. Such facts bring us back 
once more to the “constitution ” of the plant. Some, from this 
point of view, are sound, others are wanting, and we look 
forward to the time when we shall be able to say why this 
should be so and give those who are responsible for our 
crops further aid in, what is rapidly becoming a pressing 
problem, the raising of disease-resistant races. 
