56 NATURAL AND ARTIFICIAL INFECTION. 



also occur amongst species of fungi living above ground. Thus 

 the mycelium of Botrytis spreads from plant to plant, and on seed- 

 lings in hot-beds, may form felted masses. Similarly the mycelia 

 of Erysipheae, of Trichosphaeria, and of fferpotrichia make their 

 way from one part of a plant to a neighbouring part in contact. 



Artificial infection may be carried out by means of spores 

 or by mycelium. In the case of swarm-spores, the operation 

 can only be conducted in a damp chamber and on well- 

 moistened leaves. Thus, young plants of beech must be well 

 sprayed, then infected with conidia of Phytophthora omnivora, 

 and placed under a bell-jar to prevent drying up. In this and 

 many other similar cases one finds that while the spores require 

 moisture to ensure germination, yet the germ-tubes easily leave 

 the water-drops and penetrate into the leaves ; in other words, 

 the hving leaf exerts a greater influence on them than the water, 

 the chemotropic stimulus is stronger than the hydrotropic. 



The spores of the lower fungi are best isolated by the aid 

 of a lens or microscope, then washed on to the place to be 

 infected. In the case of Ustilagineae and Uredineae the same 

 method is used, except that dry powdery forms of spore are 

 simply dusted on to the host-plant to be infected. When spores 

 of Ustilagineae are being used the addition of excrement of 

 some sort is frequently of advantage, since it promotes better 

 germination and the formation of conidia capable of infection 

 after it is exhausted. One must also pay attention to the fact 

 that some smut-spores can only infect the base of the stem or 

 parts in process of elongation, while others can only attack 

 parts of the flowers. The teleutospores of the Uredineae must 

 first be germinated in order to obtain the sporidia with which 

 infection is carried out ; this generally takes place in water. 

 Thus with species of Gymnosporangium it will be found best to 

 mix the whole gelatinous mass of teleutospores with a little 

 water in a shallow glass dish, and to ascertain, by microscopic 

 investigation after a few hours, whether any sporidia have been 

 produced. If this be the case, the gelatinous mass is thoroughly 

 broken up, more water added, and the yellowish water sprinkled 

 over the host-plant. Care must, however, be taken that the 

 larger portions of the teleutospore-mass are not left on the 

 leaves, otherwise death of the latter will occur at these places 

 without infection taking place. For a similar reason it is not 



