HOW FUNGUS DISEASES ARE DISSEMINATED 15 



the leaf of a plant covered with mildew and allowed to crawl 

 about for some little time, and is then placed on another leaf 

 of the same kind free from disease, and again allowed to 

 crawl about, within two or three days the track made by the 

 slug on the previously healthy leaf will be covered with mil- 

 dew. The slimy surface of the slug picked up spores from 

 the infected leaf, and in moving about, deposited them on the 

 healthy leaf. This is what slugs do constantly of their own 

 accord. As specific instances of the unconscious transporta- 

 tion of fungus spores by insects, the following may be noted : 

 The well-known ' stinkhorn ' {Phallus impudicus), in com- 

 mon with all the members of the family to which it belongs, 

 has become specially adapted for the purpose of utilising 

 flies as agents in dispersing its spores. When mature the 

 fungus emits a very strong and, from the human standpoint, 

 exceedingly offensive smell. The very minute spores are im- 

 bedded in a green semi-liquid mucus which has a very sweet 

 taste, and is produced in such quantity that it drips from the 

 fungus if not removed. Attracted by the strong smell, 

 numerous flies assemble and feed greedily on the spore-laden 

 sweet mucus. By this arrangement the spores are not only 

 removed in immense quantities on the feet and proboscides 

 of the flies, but a copious growth of mycelium has been 

 obtained from the dung of flies that had been fed on the 

 mucus. 



The dangerous fungus parasite called 'ergot' (C/avicefis 

 purpurea), which grows from the grain of rye, wheat, and 

 many other grasses, has two forms of spores. The summer 

 spores produced on black, horn-shaped bodies, springing from 

 the grain, are exceedingly minute, and are imbedded in a 

 quantity of sweet mucus. This sweet substance forms the 

 food of certain kinds of flies, who, in visiting healthy grass- 

 flowers, leave some of the spores adhering to their proboscides 

 on the stigmas, thus securing infection. 



The fungus causing 'apple canker' {Nectria ditissima) is a 

 wound-fungus, that is, its spores cannot effect an entry into 

 the unbroken surface of a plant, but only through wounds 

 caused by some other agent. The Nectria is an indigenous 

 fungus, and has always been with us, and in all probability 

 always will be, but during late years it has become much more 

 aggressive, and epidemics of ' canker ' are too frequent at 

 present. This rapid spread of 'canker' coincides with the 

 introduction and spread of the ' American blight ' or ' woolly 



