Minnesota Plant Diseases. ^>i 



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: ore raised in caves from beds of manure. Many molds and 

 any black knot allies are also constantly found upon dung, 

 nimals are thus very potent factors in the distribution of the 

 spores of such fungi. Woodpeckers play the same role as do 

 squirrels, for they open holes in the bark of trees by which 

 fungi gain entrance to a tree and boring insects are similar 

 abettors in the attack of wound parasites. 



Man is an important agent in spore distribution. In all 

 of his commercial transactions, such as shipments of grains, 

 introduction of plants and moving of commodities, fungi of 

 many kinds may be introduced and spread over vast areas. 

 The mallow rust furnishes a good example. This rust was in- 

 troduced from South America less than twenty-five years ago 

 and has since spread over nearly the whole world, becoming a 

 great destroyer of many kinds of plants of the mallow family. 

 The spores of many fungi lurk on the seeds of other plant 

 parts and develop with the advent of favorable conditions. 

 Smut spores are very good examples of such fungi for they 

 often attach themselves to grains of grasses and when the 

 grain germinates attain conditions favorable for their germi- 

 nation. This, at the same time, is precisely the best period 

 for the infection of the grain-plant. Fungus spores or mycelia 

 may be present in bulbs or underground parts of plants which 

 are transported from one place to another and thus are spread 

 over wider areas. 



Explosive apparatus. In addition to these external factors 

 of wind, insects, etc., some fungi have developed special meth- 

 ods of their own for hurling out spores so that these shall be 

 scattered over larger areas or may better be caught by the 

 wind. The cup fungi have one such device. Here the spores 

 are formed in long cylindrical sacs. These sacs have lids at 

 the apex and when ripe throw off the lids arid spurt out the 

 spores, together with a little drop of fluid material. Often 

 numerous sacs blow off at once so that one sees little clouds of 

 spores arise from the cup. The little mold fungus which 

 causes fly cholera has also a shooting device to thrust off its 

 spores. Each little spore is formed on the end of a thread and 

 is finally snapped off by the protusion of the wall just under- 

 neath the spore. One sees these spores as a little halo around 



