FUNGI. 337 
covery of dimorphism, or the phenomenon of the 
alternation of generations among fungi. The well- 
known orange-red spots so common on the leaves of 
the berberry are but one form of the yellow rust 
of wheat and other cereal crops. If the leaves of 
the berberry are inoculated with the spores of 
the wheat-rust, they will produce the Atcidium 
or berberry-blight, figured above; and if the 
spores of the AXcidium are sown on the leaves of 
wheat, they will produce the wheat-rust. But, 
strange to say, if the spores of either form be 
sown on the plant on which it is itself parasitic, 
they fail altogether to produce the same plant ; 
and this alternation of generations will account for 
the fact, so long considered mysterious, that rust is 
apt to appear not in successive but in alternate 
years on the same crop. The class of fungi to 
which the berberry-blight belongs is exceedingly 
beautiful, and abounds in most interesting objects 
for low powers of the microscope. 
Many fungi, contrary to the habits of the race, 
seem to live on mineral matter. Numerous exo- 
tic Polypori, for instance, grow on hard volcanic 
tufa, without a particle of organic matter. Other 
fungi are not unfrequently found in this country 
growing in abundance on the hardest gravel stones, 
and bare plastered walls destitute of all organic 
nourishment. Mr. Ivor found a Didymium on a 
Y 
