THE PARASITIC EXOASCEAE. 145 



The presence of a perennating mycelium is the cause of 

 many so-called " witches' brooms " on woody plants. In fact, 

 the majority of the structures known by that name are caused 

 by species of Exoasmus, though these of barberry, silver fir, 

 acacia, and buckthorn, are due to Uredineae, and others are 

 ascribed to mites (Phytoptus). 



" Witches' Brooms " (Hexenbesen) are bushy growths, which 

 remind one at first sight of stranger-plants growing, like 

 mistletoe, on the branches of other plants. They generally 

 ■originate from a bud which has been infected during the previous 

 summer, either directly or through its subtending leaf. This bud 

 produces a twig capable of abnormally increased growth, most of 

 its sleeping buds are developed into branches, and the whole 

 .system shows marked negative geotropism. (See Fig. 3). . The 

 spores of the fungus are produced on the leaves of the broom. 

 The characteristic features of a witches' broom are : that, 

 without regard to the direction of the branch on which it is 

 borne, it is negatively geotropic in a marked degree, and 

 endeavours to develop like a terminal leader shoot ; that the 

 point of infection is distinctly conspicuous as the starting point 

 of the broom. Sadebeck regards any twig-hypertrophy as a 

 witches' broom, even that of JSxoascus Tosquinetii where there 

 is no basal swelling and the twigs exhibit only very slight 

 negative geotropism. 



The forms of witches' brooms are very varied. Amongst 

 the best known are the hanging broom-like masses developed 

 from buds of the leader shoots {e.g. on cherry trees). As a 

 result of the rich growth of twigs and their premature death, 

 many of these brooms become tangled nest-like structures. The 

 twigs in some are much elongated, in others shortened, in 

 every case, however, they are abnormally numerous. As a rule 

 the original leader shoot, on which some lateral bud has developed 

 into a witches' broom, shrivels up and dies, its contents being, 

 as it were, absorbed by the hypertrophied branches. Other 

 general features have already been discussed in Part 1. of 

 this book. 



Smith^ found that the form of the witches' broom is not 

 determined exclusively by the fungus. The perennating my- 

 ..celium indeed gives the first impetus towards its formation, 



^ Smith, loc. cit. 

 K 



