56 Minnesota Plant Diseases, 



cherry trees. Almost any kind of cherry is subject to broom- 

 ing. The broom branches are usually very numerous and the 

 leaves do not acquire the usual regularity of form but are often 

 distorted. This is especially noticeable where the fungus part- 

 ner forms its spores. The latter are produced usually on the 

 lower surfaces of the leaves over which a grayish filmy coat 

 spreads. This fungus belongs to the sac fungi and is a relative 

 of the cup fungi. Although it produces no cups yet the spores 

 and sacs are arranged in a similar fashion. Birch trees some- 

 times carry brooms which are caused by fungi of the same 

 group. 



Witches'-brooms are not always caused by fungus attacks. 

 Insects are sometimes also able to produce them, but in many 

 cases the origin of the broom is unknown. In the latter 

 category stand the brooms which are sometimes found on pine 

 trees, occasionally attaining a diameter of over ten feet. 



Other examples of individuation. Witches'-brooms are not 

 the only cases of the building up of a physiological individual 

 from a fungus and a part of its host. Swellings are often pro- 

 duced on pine trunks which are five times the thickness of the 

 adjacent part of the trunk. Such swellings are caused by a 

 parasitic fungus and may be considered again as individuals 

 which are living luxuriantly at the expense of the host tree. 

 Burls on trees are by no means uncommon though their origin 

 from a fungus infection is not always clear. In many cases 

 their origin is unknown. Again, such tubercular swellings as 

 are found on Indian corn, where smut later arises, are in reality 

 favored individualized parts of the corn with a fungus partner. 

 Large galls form in allies of the blueberry plant upon leaves 

 and stem, and these galls are also of fungus origin. A larger 

 number of examples might be cited but enough have been 

 mentioned to illustrate the individuation of fungus with plant 

 parts in a parasitism which is ultimately detrimental to the host 

 plant. In most cases of parasitism of fungi the host plant does 

 not in any way show a stimulation of the affected parts and the 

 absence of any difference in the action of the affected and nor- 

 mal parts indicates a low degree or absence of individuation. 

 It is worthy of mention here, however, that parasitic fungi in 

 general thrive best on healthy plants rather than on weaklings, 



