258 



The Spread of Fungus Diseases. [aug., 



that plant is infected for all time. The discovery of this means 

 of perpetuating a disease by mycelium present in the reproduc- 

 tive portion of the host-plant fully explains the failure to check 

 certain diseases by spraying or other measures, where the idea 

 was to destroy spores or prevent their entering the plant on 

 germination. The same discovery also strongly suggests the 

 probability that many sudden outbreaks of fungus-epidemics, 

 attributed to the rapid production and diffusion of spores, may 

 not in reality be due to spores at all, but to the presence of 

 hybernating mycelium which, favoured by certain weather 

 conditions, extends rapidly through the host-plant and causes an 

 epidemic. 



Infection by means of spores dispersed by wind, insects, or 

 other external agents, is the oldest and most general method by 

 which fungi maintain their continuance in time, and also extend 

 the area of their distribution. The more specialized and less 

 general method of securing these two objects, by means of 

 hybernating mycelium passing directly from one generation of 

 the host-plant to another is equally certain in its results, and at 

 the same time more economical ; when the balance between 

 fungus and host-plant is fully established, as in the rye-grasses^ 

 the production of spores is no longer necessary. 



The following illustrations of infection and dispersal of fungi, 

 by means of hybernating mycelium in the reproductive portion 

 of the host-plant, furnish examples of the various phases in the 

 evolution of this method, which may be grouped under three 

 heads. 



1. The simplest or least differentiated stage, where both spores 

 and hybernating mycelium in the seed are necessary. 



This illustrates the most primitive attempt to perpetuate a 

 species by means of hybernating mycelium ; the older method by 

 means of spores is still necessary, and plays an indispensable 

 part in the process. 



Examples. — " Smut " caused by species of Ustilago, in some 

 cereals and other grasses. 



2. Hybernating mycelium alone is capable of perpetuating the 

 parasite from year to yean Spores however continue to be 

 produced, and by infecting other host-plants extend the 

 geographical area of the fungus. 



