FUNGAL PARASITES 175 
furnish another example of remarkable inter- 
ference with the ordinary growth-processes 
of the host plant that a parasite is able to 
induce. The wild cherry trees are particu- 
larly subject to the attacks of a fungus 
(Exoascus) which fruits within the leaves 
and alters the boughs affected by it in a 
curious manner. They are much more freely 
branched, the leaves are often smaller and 
sometimes deformed, and flowers are seldom 
or never produced on the affected parts. 
Another kind of witches’-broom occurs on the 
fir trees in continental forests, though they 
are not so frequently seen in this country. 
It is produced by one of the rust fungi 
(Acidium elatinum). <A twig affected with it 
is a striking object, inasmuch as it grows up 
vertically on the bough instead of horizontally. 
This erect habit is maintained, and as the 
years pass the witches’-broom comes to 
resemble a little Christmas tree arising from 
an ordinary, horizontally-growing bough of 
the fir tree. Several kinds of firs are liable 
to the attacks of this fungus, but it is on the 
silver fir that the witches’-broom is most often 
seen. 
Such relations between fungus and host as 
those just described, and many other examples 
might be added, very clearly prove that these 
apparently simple parasites are remarkably 
complicated from a physiological point of view. 
The surprising thing about them is their very 
accurate degree of specialisation to the hosts, 
