46 Minnesota Plant Life. 



merged in the water, although the majority are parasitic upon 

 living plants or animals, or make the dead bodies of these their 

 habitat. The life of a fish-mould differs from that of the black 

 mould in some important particulars from its being an aquatic 

 organism. Its spores are not mere passive spheres of micro- 

 scopic size like those of the black mould, but are provided with 

 swimming lashes, so that they may propel themselves through 

 the water in search of other fish or insects upon the bodies of 

 which they may be fortunate enough to obtain a lodgment. One 

 fungus related to the fish-moulds, not yet discovered in Minne- 

 sota, but possibly occurring somewhere within the state, is 

 noteworthy in botanical annals as being the only fungus known 

 to produce motile spermatozoids like those of the algae and, as 

 will be seen later, of the ferns and mosses. 



Mildews. Closely related to the fish-moulds are the mildews. 

 These are fungi which live as parasites upon land plants. A 

 striking example of the group is the mildew of mustards, which 

 occasions a rotting of the stems and leaves in the shepherd's- 

 purse. Another causes a rotting of potato tops and is one of the 

 most serious diseases of the potato with which cultivators have 

 to contend. Still another which occurs in Minnesota is the 

 mildew of the grape-vines, inducing the leaves to wither and 

 decay. The lives of mildews are in many respects similar to 

 those of the fish-moulds, but with certain differences owing to 

 their non-aquatic habits. For example, the mildew of the vine 

 when it attacks the leaves goes about the task somewhat in this 

 fashion. From the air, into which from other mildewed leaves 

 the spores have been projected, certain spores come to fall upon 

 the surface of a healthy leaf. They lie upon the skin of the leaf 

 and extend little infecting tubes which crawl around upon the 

 surface of the leaf until they find one of the air-pores with which 

 the leaf is provided for respiratory and vapor-excretory pur- 

 poses of its own. Into such apertures the fungus insinuates 

 itself, and as its infection-tube crawls beneath the skin of the 

 leaf among the soft cells filled with leaf-green and starch, it 

 finds there plenty of food material. Into each cell it drives a 

 little sucking organ and extracts the nutritive substances and 

 converts them into its own body. Going thus from cell to cell 

 it finally saps the life of so many of them that the usefulness of 



