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 

 (JEcidium 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, 



