62 Minnesota Plant Diseases. 



thread which in many cases grows directly to a ventilating pore 

 of the leaf and enters through this pore. In many cases, how- 

 ever, it is able to bore its way through the walls and thus pene- 

 trate to the interior. Special threads may be developed to 

 fasten the germinating spore to the host plant. This is accom- 

 plished by minute disk-like ends similar to little rubber vacuum 

 cups. Abundant hairs on the surface of a host plant, or a very 

 thick cuticle, may lessen the danger to the host of fungus at- 

 tacks. Some fungi select certain periods in the growth of the 

 host during which the latter is less able to ward off the attack. 

 Such is seen in the oat smut and damping-ofJf fungi, which at- 

 tack seedlings, or again in the wounded trees where a fungus 

 gains entrance before the tree has had time to close a wound in 

 the normal manner. Since the selection for a time for attack 

 is in many cases of great importance to the fungus, the latter 

 usually forms its spores to coincide with this favorable time. 

 Fungi which attack the trunks of trees are usually unable to 

 penetrate the bark unless aided by wounds of some sort, but 

 they may occasionally penetrate through the ventilating holes. 

 Mention has already been made of those fungi which require 

 previous preparation for attack, as is the case in many of the 

 wound parasites. 



The living together with special plant-parts. Parasitic fungi 

 do not usually live together with all parts of their host plants 

 but confine themselves to certain organs, or at least show pref- 

 erence for certain plant-parts. 



Leaf-inhabiting parasites. Perhaps the most conspicuous 

 and common are those which prefer the partnership with 

 leaves. Most .rust fungi are of this class; most mildews, 

 blights, leaf-curls and that great group of imperfectly known 

 fungi which commonly form the so-called leaf spots. The 

 foliage leaf is usually selected by the fungus, though more 

 rarely the scale leaves or floral leaves may also be attacked. 

 Sometimes, as in the leaf-spot, the fungus only inhabits a small 

 portion ; in others it may pervade or cover the whole leaf. The 

 leaf-dwelling parasites are perhaps the most destructive of all 

 fungus parasites, both on account of their number and their 

 effect upon the starch-making machinery of the plant. The 

 ease with which the fungi develop in the leaf-tissues is perhaps 



