26 Minnesota Plant Diseases. 



ber of such imperfectly known fungi, many of them being of 

 great economic importance. They are usually designated as 

 "Imperfect Fungi" and are classified temporarily according to 

 a very artificial system under what are usually termed "form 

 genera." 



This selection of fungi for special substances, for the pro- 

 duction of certain spores, and the production of different spores 

 according to seasons, has given rise to a very remarkable phe- 

 nomenon in the succession of spores. Not only may some 

 parasitic fungi form different kinds of spores but these spores 

 may be formed on widely different plants. The wheat rusts 

 furnish us with the most familiar examples. The spring spores 

 are formed on barberry or on buckthorn, or on some other 

 plant, according to the kind of rust, while the summer and 

 winter spores are formed on grasses. If one sows spring 

 spores on barberry they will not develop but they must be con- 

 veyed to a grass plant before infection takes place. In a simi- 

 lar manner, when the winter spores germinate, the little spores 

 which are produced on the germ threads must be borne to a 

 barberry or buckthorn leaf before they can cause infection. 



Spore distribution. Just as the seed plants utilize many 

 agencies for the purpose of distribviting their seeds over as 

 wide an area as possible, just so do fungi utilize the same agen- 

 cies for the dissemination of their spores. The fungi in gen- 

 eral may be said to be very prodigal of their spores, so that 

 these are produced usually in great numbers. This may be 

 accounted for in the peculiar requirements of the spore for ger- 

 mination and for further development. Hund-reds or thou- 

 sands usually fail to develop where one or two find suitable 

 conditions and give rise to a mature plant. A mushroom or a 

 puff-ball produces literally millions of spores, yet from the 

 abundance of any given kind we know that very few germinate 

 and come to maturity. Again spores are microscopically small 

 and hence in general cannot contain a great deal of nourish- 

 ment. They cannot therefore withstand unfavorable condi- 

 tions of germination for such protracted periods as can many 

 seeds of seed-bearing plants. Many spores can pass through a 

 very long resting period and are capable of germination at the 

 end of this period, but after germination is begun the spore 



