180 SECTIONAL ADDRESSES 
The extreme dissimilarity in systematic relationship between the teleuto- 
spore and zcidiospore hosts in the hetercecious Rusts is very striking. 
The metabolism of the dissimilar hosts is probably vastly different, yet 
the fungus flourishes during both its alternating phases. ‘The divergence 
between the teleutospore and ecidiospore hosts cannot be wholly bound 
up with the usual stomatal infection of the former and cuticular penetra- 
tion of the latter, for unless there are important metabolic differences 
between the two phases of the fungus it is difficult to understand why the 
zcidiospores do not infect, by way of the stomata, the host on which they 
are produced since they infect the alternate host in this manner. It may 
be thought that change of host is correlated with an alteration of the fungus 
from the haploid to the diploid condition, or vice versa, but so far as the 
sexual processes of the few hetercecious Ascomycetes are understood this 
does not apply, nor does the correlation hold good for the hetercecious 
worms among animal parasites. If we try to understand hetercecism on 
a genic basis we may perhaps suppose that in a hetercecious fungus there 
are two genes or sets of genes controlling metabolism which become 
separately active during the different phases of the life-cycle. It is easier 
to visualise hetercecism arising suddenly by a large change than to con- 
ceive of its evolution by a succession of minute variations. In connection 
with hetercecism in the Rust Fungi further information is required as to 
what happens when zcidiospores and sporidia are placed on the hosts 
from which they have been derived and also on other plants which play 
no part in the perpetuation of these fungi, comparable with the knowledge 
which is available about the behaviour of uredospores on inappropriate 
hosts. Gibson ** shewed some years ago that uredospores germinated 
normally on inappropriate hosts and formed appressoria over the stomata 
and vesicles below the stomata, from which hyphe grew out ; these, how- 
ever, were unable to establish haustoria in the mesophyll cells and speedily 
died. Mrs. Hanes ** has recently shewn that on inappropriate hosts 
which are fairly closely related to the proper ones the behaviour of the 
germinating uredospores is sometimes of another kind. Inthe cereal rusts, 
for instance, if uredospores are placed on leaves of the wrong cereal 
the initiation of infection is normal, but at a slightly later stage there is 
sometimes a violent reaction between parasite and host which leads to the 
death of the mesophyll cells involved ; the fungus then makes no further 
progress in the tissues. Such behaviour is comparable with the well- 
known hypersensitiveness of resistant host varieties to specific rust fungi. 
With the Powdery Mildews (Erysiphaceae) Corner 4 has shewn that 
on inappropriate hosts the early stages of penetration are the same as on 
the proper host, i.e. the cuticle is pierced mechanically by a stylar process 
which in its passage through the cellulose part of the wall is preceded by 
a local swelling of the latter, the papilla. ‘The penetration process, how- 
ever, usually develops no further in the inappropriate host and is probably 
killed by toxic substances in the ‘ host ’ cell. 
In contrast to the extreme specialisation of obligately parasitic fungi to 
32 New Phytologist, 3, p. 184 (1904). 
33 Cambridge Ph.D. thesis, 1933 (not yet published). 
34 New Phytologist, 34, p. 180 (1935). 
