Observations and Experiments on Cereal Rusts. 155 
(6) Discussion of “ Mycoplasm hypothesis.” 
At this stage the writer may refer to other observations which 
were made on some of the most susceptible varieties of wheat 
and barley sown in the open last autumn (1921). 
I. Seedlings of Red Sudan wheat raised in the laboratory 
were transplanted in a small plot outside the cages on August 
24th. Some of the seedlings were transplanted to a bed outside 
the laboratory. 
2. Seedlings of a Mesopotamian variety of barley, exceedingly 
susceptible to yellow rust, were transplanted outside the cages 
and also outside the laboratory. 
3. Several self-sown plants of barley and wheat were trans- 
planted on October 17th from the cages to the beds outside. 
There was plenty of mildew, but no rust on these plants. 
All three sets of plants were frequently examined for the 
appearance of yellow rust under conditions most unfavourable 
for an infection from without, because of the almost complete 
absence, as far as could be seen, of yellow rust around Cambridge. 
Up to the end of May 1922 no trace of yellow rust was 
observed on any of these plants. Occasionally a few pustules 
of P. triticina on wheat and P. simplex on barley were noticed 
on plants transplanted outside the cages. Brown rusts both on 
barley and wheat, as has already been pointed out, were present 
on self-sown plants from the previous harvest (1921). Again, 
as remarked above, while dealing with the incidence of brown 
rust, Luther Burbank wheat growing at the Botanic garden all 
along (Sept. 192r to July 1922) showed uredo-sori of that rust, 
but not even once was yellow rust noticed on that plot up to 
the end of June 1922 (a period of over ten months). 
It is difficult to understand why yellow rust should not have 
appeared on the winter crops, on the wheat at the Botanic 
garden, and even on plants of varieties known to be exceedingly 
susceptible, if there was anything like an internal germ of 
disease in existence. It is essential in this connection to point 
out that the variety of Mesopotamian barley referred to above 
was very badly smitten with yellow rust during the summer of 
Ig2t. . 
Plants of Red Sudan wheat and barley had been exposed in 
the open at the University farm for more than ten months, 
without showing a pustule of yellow rust. 
To make this point still more forcible, it may be mentioned 
that wheat artificially inoculated with yellow rust on Oct. 26th, 
Ig2I, bore pustules in less than three weeks’ time, so there 
could be little doubt about the weather being favourable for 
the internal germ of disease to manifest itself if it did exist. 
