INJURIES INDUCED BY PLANTS 45 



organs of vegetative propagation, produce the same plant-form 

 as that from which they themselves sprung, are called gonidia. 

 Following the example of De Bary, this term may be taken to 

 replace that of conidia, introduced by Fries. 



The gonidia serve chiefly for the rapid propagation of 

 fungi during the growing season, whereas, in general, the 

 sexual spores serve to carry the species over from one year 

 to another. 



I may here briefly sketch the mode and conditions of life 

 of the fungi. Just as in the case of phanerogams the ger- 

 mination of seeds, and the length of time during which they 

 will retain their vitality, are much influenced by external 

 factors, so in the case of spores and gonidia the power to 

 germinate varying with the different species appears either 

 immediately after ripening, or not till after the lapse of a long 

 period of rest. 



On the other hand, in the case, for instance, of the gonidia of 

 the rust-fungi, the power to germinate is lost a few days after 

 ripening, whereas the oospores of *Phytophthora omnivora may 

 remain dormant in the ground for at least four years, without 

 losing their vitality. 



The demands as to heat are not so great as in the case of the 

 higher plants, and thus it is that we see the most luxuriant 

 fungus-vegetation in autumn, at a time when the growth of trees 

 has ceased. The optimum temperature for fungi, as for other 

 plants, varies very much, but in this connection we still await the 

 results of reliable investigations. In the case of those fungi 

 which concern us here a temperature over 212 F. (100 C.) is 

 undoubtedly always fatal. 



One vital condition, of extreme importance for fungi, is a high 

 degree of humidity of the air or of the substratum in which they 

 develop. This is due not only to their requiring large quantities 

 of water, but also, and much more, to the ease with which the 

 mycelia or young sporophores die in a dry medium from the 

 effects of excessive evaporation. On this account it is very 

 seldom possible for the mycelium to develop in the open air. 

 It is for this reason also that in all the rusts and smuts, and even in 

 the case of a great number of Discomycetes, the sporophores 

 which usually require to scatter their spores outside the plant 



