INJURIES INDUCED BY PLANTS 49 



induce the outbreak of an epidemic. Here a few examples may 

 be cited in illustration of this point. 



Phytophthora omnivora produces spores (in this case called 

 oospores) in the interior of the seedling, as the result of sexual 

 fertilization. When the plants decay, these spores get into the 

 ground, where they may rest for a series of years and produce 

 the disease afresh, should the right kind of seedlings be present. 

 But, in addition to these oospores, the parasite produces numerous 

 gonidia on the surface of its leaves. These are capable of 

 germinating at once, and are blown by the wind, or conveyed by 

 animals or men, to plants in the neighbourhood, the result being 

 the formation of new hotbeds of infection. 



In the case of Trametes radiciperda, which, on the spruce at 

 least, almost always produces its sporophores in holes in the 

 ground, new centres of infection are usually established by spores 

 that have been distributed by mice. 



The smut of wheat is generally induced by employing seedj to 

 the outside of which spores of the smut-fungus have adhered, 

 but it may also be caused by manuring with fold dung if infected 

 straw has been used as litter. 



The conditions become most interesting in the case of heter- 

 cecious rust-fungi that is to say, parasitic fungi which complete 

 the various phases of their development not on the same plant 

 but on two different species. Here mention need only be made 

 of the connection between the fungus of the barberry and rust of 

 wheat, or between sEcidiiim abietinum and Chrysomyxa Rhodo- 

 dendri and Chrysomyxa^Ledi, or, finally, between jEcidium 

 columnare and Melampsora Goeppertiana. In the case of these 

 parasites the occurrence of the disease depends on the presence 

 of both host-plants : still De Bary has demonstrated that in cases 

 of necessity Chrysomyxa Rhododendri may exist without spruces, 

 and it appears to me to be beyond doubt that Melampsora 

 Goeppertiana is able to develop without the silver fir. We know 

 only one or other of the stages of development of a series of 

 rust-fungi, and it remains to be determined what the other fungus- 

 forms are with which they stand in relationship. 



The method of attack of parasites, also, reveals the most 

 marked differences. Whereas the epiphytes whose mycelium 

 vegetates externally on the epidermis of leaves, fruits, and stems 

 send only delicate absorbing organs into the interior of the 



E 



