EXOASCACEAE 137 



thus producing numerous, minute, secondary spores which 

 completely fill the ascus. 



The two most important genera, Exoascus and Taphrina, 

 are parasites, and in many instances do a considerable 

 amount of damage, causing distortion of leaves or fruit, or 

 forming dense tufts of branches known as ' witches' brooms ' 

 or 'crows' nests,' from their general appearance. These 

 structures are characterised by a dense tuft of branches 

 springing from a single, more or less swollen point on the 

 branch from which they originate. The branches of a broom 

 are usually more or less thickened and densely branched, and 

 grow in a vertical direction, hence such tufts are very con- 

 spicuous, springing as they usually do from more or less 

 horizontal branches. Brooms never produce flowers, and the 

 leaves are often much modified, being thicker in substance 

 than normal leaves, contain less chlorophyll, and are often 

 more or less crumpled or distorted. Asci are produced on 

 the leaves, and the mycelium hibernates in the shoots. 

 Brooms sometimes assume large proportions, and add to 

 their size year by year. 



It is important to bear in mind the fact that witches' 

 brooms are in some instances produced by members of the 

 Uredinaceae or rusts, in others by mites. Not infrequently 

 brooms, caused by fungi and mites respectively, occur on the 

 same tree, as in the common birch, or on the same tree, both 

 mites and fungi may be present on the same broom. 



The distinction between the genera Exoascus and Taphrina 

 is based entirely on biological grounds, there being no 

 marked morphological distinction between the two. 



In Exoascus the mycelium is perennial in the young shoots 

 of the host-plant, consequently there are two modes of repro- 

 duction : (1) by resting mycelium in the branches, which 

 passes into the buds, and finally develops in the leaves or 

 fruit; (2) by means of spores which are distributed by wind, 

 birds, insects, etc., thus enabling the fungus to attack new 

 hosts and extend its area of distribution. 



In the genus Taphrina, the mycelium is only annual in 

 duration, hence infection by spores is the only means of 

 perpetuating the species from year to year. 



The species in this group, apart from the host-plant, are 

 not very clearly marked. 



Massee, Geo., British Fungus-Flora, 4. 



