Minnesota Plant Diseases. 63 



explained by the fact that, on account of the great air-spaces 

 inside of the leaf, the fungus can easily obtain the air gases 

 which are necessary for its development; secondly, that 

 these spaces are always more or less moist on account of the 

 water vapor given off by the leaf tissues ; and, lastly, they are 

 not too highly illuminated by sunlight. The interior of a leaf, 

 therefore, furnishes excellent opportunities for fungus develop- 

 ment and many fungi have availed themselves of these oppor- 

 tunities. Moreover, the fungus can often gain easy entrance 

 through the air pores on the leaf surface. When a leaf-fungus 

 dwelling in the interior is about to produce spores, it forms the 

 latter usually at the surface of the leaf. Sometimes, as in the 

 potato-blight allies, it shoves the spore-bearing threads out 

 through the air pores, but this is not the method in the rusts 

 Here the leaf surface is broken open, splits out, and the interior- 

 ly formed spores reach the air through the split. Of course 

 such an injury, minute though it may be, really injures the leaf 

 by the interference with the leaf control of water vapor. A 

 large number of such wounds, together with the injury by loss 

 of nutrition to the fungi, may cause the death of the leaf. The 

 spores of the fungi usually appear as a powder or cake uncov- 

 ered by the upheaval of the leaf surface tissues. In some few 

 leaf-fungi the spores are formed internally and are only released 

 by the decay of the leaf. 



Stem-inhabiting parasites. The stems of plants furnish an- 

 other favorite abode for parasitic fungi. In the stems of herb- 

 like plants the fungus problems of entrance and life are not 

 very different from those of the leaf except that the tissues are 

 firmer. Hence we find many rusts capable of living either on 

 the leaves or stems of a given host plant. But in woody plants, 

 as in shrubs and trees, the fungus meets with new difficulties 

 in the nature of a thick layer of bark which must be penetrated 

 before the living part of the stem can be reached. Moreover, 

 the compactness of the tissues and the resultant absence of 

 larger air-spaces do not make the stem such a congenial dwell- 

 ing place as is the leaf. In such woody stems, therefore, we 

 find almost exclusively those fungi which are capable of break- 

 ing down woody tissues and feeding on them. It has already 

 been stated that these fungi must usually depend for entrance 



