DISEASE CAUSED BY FUNGI 



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or very strongly negatively geotropic, whereas the parent 

 branch from which it originates is horizontal in conifers, 

 and more or less so in other trees. This erect habit 

 causes the witches' broom to stand out conspicuously, 

 and to resemble an alien. All the branches of the broom 

 are permeated with the mycelium of the fungus through- 

 out. The leaves are always annual in duration, small in 

 size as compared with those of the parent tree, almost 

 devoid of chlorophyll, and bear the aecidial or ' cluster- 

 cup : fruit of the fungus. After the spores of the fungus 

 are shed, the leaves fall, quite early in the season. No 

 fruit is produced by a witches' broom. Owing to the 

 absence of chlorophyll a witches' broom is unable to 

 supply itself with food, and is practically a parasite on the 

 parent plant to the same extent, or even more so than a 

 mistletoe would be. The broom and its parasite are very 

 nearly in a symbiotic condition, the fungus derives its food 

 from the broom, and the latter in turn depends for its food 

 on the stimulation exercised by the fungus on the parent 

 plant. Symbiosis is not yet complete, for although there 

 is mutual benefit, and the compound structure may con- 

 tinue growing for many years, the fungus is yet dominant, 

 as the host is modified adversely to the extent of not being 

 able to reproduce itself. 



It may be well to state that many witches' brooms, in- 

 distinguishable so far as general appearance goes from 

 those described above, owe their origin to members of the 

 animal kingdom, mostly microscopic mites belonging to 

 the genera Phytoptus, Eriophyes, etc. Such brooms are 

 common on silver birch, alder, etc. 



Diseases induced by fungi often cause very remark- 

 able modifications in form and structure of the 



