160 Transactions British Mycological Society. 
of yellow rust, but that the uredospores are on the whole very 
short lived. It has been shown that even at 5° C. (by no means 
a high temperature) after one month’s time they retain only 
20 % of their viability. Besides one finds that a considerable 
number of spores, even when removed from pustules hardly a 
week old, look quite dark under the microscope and show a 
complete absence of the characteristic orange coloured contents. 
Such spores do not germinate at all. Undoubtedly the uredo- 
spores of this rust keep better at lower temperatures. 
The uredospores of brown rust are longer lived than those of 
the yellow one and also keep better than them at warmer 
temperatures. 
It may be pointed out in this connection that considerable 
difficulty was encountered in keeping the culture of yellow rust 
in the summer of 192I, as it was abnormally warm. On account 
of the frequent failure of infection the pure culture that had 
been kept going since Nov. 1920 was lost. Moreover all attempts 
to establish a new one were unsuccessful. There was occasionally 
a little infection after a prolonged incubation but it seldom gave 
enough material for re-infection. The impaired condition of the 
uredospores and their subsequent failure to infect the host even 
when protected from direct sunlight suggests that conditions for 
the fresh infection of wheat and other cereals were very unfavour- 
able in the summer of 1921. On account of the exceptionally high 
temperature (the average maximum in the shade during July 
was as much as 26:2°C.) it is probable that the uredospores 
suffered great injury. It is no wonder therefore that the rust 
was practically absent during the autumn and winter of 
Ig2I-22, whereas brown rust was noted on wheat and rye 
throughout that period. The occurrence of P. simplex on barley 
and of black rust on Couch in the autumn of 1921 has already 
been recorded. 
It is interesting to note that with additional precautions it 
was possible to keep the cultures of both brown and black rusts 
going even during the summer of 1921. 
7. INCUBATION PERIODS. 
As remarked above Eriksson* has allowed a very variable 
time limit for the ‘““mycoplasm” to assume a visible form. 
While writing about one definite season he states, “the period 
lacking mycelium in the wheat plant’s life must be short, e.g. 
two to three weeks in October after sprouting,’’ because he 
found fresh pustules of yellow rust in November. In another 
season he says that this period extended over nearly nine 
months. 
* Eriksson, J., Ann. Bot. xIx, p. 55 (1905). 
