Minnesota Plant Diseases. 



61 



that in the latter case the nutrient material must first pass 

 through the walls of the host plant before it can be taken up 

 by the fungus thread. The interior-dwelling fungi may get 

 their nourishment in several ways. In many, special sucker- 

 threads are sent into the living substance of the plant. In 

 other cases the fungus threads run between the cells of the tis- 

 sues without ever coming into direct contact with the living 

 substance. On the other hand, fungi may gain entrance to 

 the cells arid live entirely within them. Such is the common 

 method of many very minute water fungi. The sucker-threads 

 of the various fungi differ considerably in shape and often fur- 

 nish important marks of distinction, since each fungus may 



FIG. i9. Infection of a tirass leaf by a rust fungus (wheat rust). Above is a summer 

 spore showing germ-tube*. Itelow it a germ-tube entering through the |>ore of the leaf 

 and is reaching down in the internal part of the leaf where it soon becomes well 

 established. After Ward. 



have its own peculiar form of sucker. The simplest are little 

 cylindrical unhranched threads. Again, they may be small 

 tubercular hyphae; others are branched to form a stubby-fin- 

 gered, hand-like system of threads. In still other cases the 

 suckers may be very much branched and the branches may be 

 coiled up into dense mats entirely filling the cell of the host. 



Methods of attack. When the spore of a parasitic fungus 

 falls on the leaf of a host, it awaits favorable conditions for fur- 

 ther development. When the moisture and temperature and 

 other conditions are most favorable the spore sends out a 



