34 



Minnesota Plant Diseases. 



very much lower temperatures. Very many spores of our 

 fungi must be able to endure forty degrees below zero Fahr. to 

 pass the winter. In fact many can undergo still lower temper- 

 atures and survive. 



Conditions of spore germination. When a spore is placed 

 under proper conditions of moisture, temperature and of other 

 factors, it germinates, i. e., grows out into a fine thread which, 

 if conditions remain favorable, develops directly into the fun- 

 gus mycelium. By far the largest majority of fungus spores 

 are capable of germination as soon as they are ripe, provided, 

 ^ of course, that such 



external conditions 

 as light, moisture, 

 etc., are favorable. 

 Many so-called rest- 

 ing spores are forced 

 to undergo a cer- 

 tain resting period 

 after maturity be- 

 fore they can germi- 

 nate. Such spores 

 are provided with 

 thick coats for pro- 

 tection. This resting period is often connected with the suc- 

 cession of seasons. For instance most of the rust winter 

 spores germinate best in the following spring and cannot be 

 made to germinate before that time. Moreover, they retain 

 but a decreasing vitality as the following summer passes, and 

 are usually incapable of growth in the fall. Such spores are 

 adapted closely to the seasons. Not only resting spores but 

 other non-resting spores may also evince such conditions. 

 Rust summer spores are generally incapable of germination 

 after the summer in which they are formed, though some are 

 probably capable of surviving the winter in vigorous condition. 

 Such adaptations are of course especially bound up in the pe- 

 culiar habits of the fungus. 



Fig. 15. — A caterpillar-fungus (Cordyceps) spore. A 

 germinating spore at different successive stages of 

 several hours apart. The small resulting mycelium 

 is seen below. Highly magnified. By the author. 



