Minnesota Plant Diseases. 19 



that if all of the air is excluded from a jar of preserves no molds 

 will develop. 



The spores of parasitic fungi will usually germinate if placed 

 in water. They often require to be kept for a certain time and 

 are often adapted for certain seasons. For instance, the black 

 rust spores of wheats and grasses usually will, not germinate 

 until the following spring. A germinating spore of a true par- 

 asite must be brought into contact with its proper host, or it 

 will soon die for lack of food. When brought into contact it 

 commences very soon its parasitic life. The spores of many 

 saprophytes, on the other hand, require nutrient substances 

 before they will germinate. It is often a matter of very exact 

 requirements as, for instance, in the case of the common com- 

 mercial mushroom. It is only within the last few years that 

 the commercial mushroom spore has been observed germinat- 

 ing. The continuation of the growth of the mycelium takes 

 place at about the temperature of the germination. Light is 

 not a necessary condition for growth, for it is not an essential 

 in the building up of starch in fungi, as it is in the case of any 

 of the leaf-green-bearing plants. Hence one finds fungi devel- 

 oping luxuriantly in caves and cellars. Light, however, some- 

 times influences the formation of the fruiting bodies. The 

 food material of most saprophytic fungi is required to be of a 

 slightly acid composition. The concentration of compounds 

 found in the nutritive substances affect profoundly the develop- 

 ment of the fungus. Certain fungi which develop well in' a 

 weak solution of sugar, cannot grow in a very concentrated 

 solution, a principle which is utilized in the preserving of fruits. 



Such fungi as the wood-inhabiting and insect-inhabiting 

 forms illustrate well the method of attack of many fungi. The 

 timber parasites and saprophytes exude from their threads a 

 chemical substance which attacks the wood tissues and destroys 

 the woody properties. The wood is thereby reduced to punk. 

 Insect-inhabiting forms exude a substance which attacks and 

 disintegrates the chitinous coverings of the insect, thus gain- 

 ing entrance for the fungus to the soft parts of the insect. 



The age of a mycelium varies considerably in different fungi. 

 Some live for but a few days, some live indefinitely, being lim- 

 ited only by the absence of nutrition, and others again are reg- 



