52 NATURAL AND ARTIFICIAL INFECTION. 



confusum and G. sahinac may, in their aecidial stage, be dis- 

 tingnished as two species inhabiting distinct hosts — Crataegns and 

 Pi/rus respectively — whereas, in their teleutospore stage on juniper, 

 they scarcely vary. In infection we have an important aid in 

 determining the host-plants of the various forms of heteroecious 

 fungi, and in this way it has been found that the same fungus 

 behaves differently according to the host-plant on which it 

 is present. Thus, in the genus Gymnosporangium, I have found 

 that a certain species had well-developed aecidia on one plant, 

 poorly developed ones on another, while on a third only spermo- 

 gonia appeared. Similarly, in that case already mentioned, 

 Hartig found the Mdnin2)sora of the aspen to produce on the 

 pine a disease of the cortex, accompanied by marked deformation, 

 while on the larch the symptoms were mere inconsi)icuous aecidia 

 on the needles. 



Amongst the Ustilagineae, experimental infection is necessary 

 to determine whether the natural infection of host-plants results 

 from germinating spores (chlamydospores), or from germinating 

 conidia (sporidia). Kiihn was able by this means to demonstrate 

 exactly that the spores of Ustilagineae produced germ-tubes 

 capable of direct infection. Brefeld succeeded in observing the 

 penetration of germinating sporidia into a host-plant. In this 

 way he proved, amongst other facts, that maize may be attacked 

 by Udilago maydis on any young part ; also, that the mycelium 

 remained local. Oats, on the other hand, could only be infected 

 by Ustilago avenae at the neck of the young seedling, and the 

 mycelium extended through the plant till it reached the inflor- 

 escence, where the spores are formed. 



In the case of the Exoasceae, two points were cleared by the 

 aid of artificial infection — the penetration of spores into leaves of 

 host-plants, and the production of witches' brooms. Sadebeck,^ 

 by means of infections of Exoascus epiphyllus on Alnus incana, has 

 produced witches' brooms artificially, thus proving that these 

 malformations really originated from the mycelium of Exoascus. 



It is by infection-experiments that one determines into which 

 part of a host the germ-tubes penetrate, whether into leaf, flower, 

 fruit, stem, or root, and also whether it passes through the 

 epidermis, or between two adjacent epidermal cells, or through 



^ Kritisclie. Untersuchunrjen iiher d. dvrch Taj^irina hervorgehrachteii Bauvi- 

 krankkeiten, 1890. 



