INJURIES INDUCED BY PLANTS 65 



no doubt that they are also brushed off and distributed by ani- 

 mals, as, for instance, by hares. The germination of the sporangia^ 

 or their production of swarm-spores as the case may be, agrees 

 generally with that of the allied species. The sporangia, how- 

 ever, reach the ground in large numbers, and are carried down to 

 the tubers by the rain-water. Should the ground remain wet, in- 

 fection follows upon the development of the germ-tube. The fact 

 that varieties of potatoes with thin skins are more easily pene- 

 trated by the germ-tube of the fungus than those having thick 

 skins may explain how it is that the latter suffer less from 

 disease. 



The formation of oospores, which I have demonstrated in 

 the case of P. omnivora, has not yet been discovered in the 

 potato fungus, and possibly it may not exist. Seeing that the 

 mycelium passes the winter in the tubers, the existence of the 

 fungus does not in this case depend on the formation of oospores. 

 The occurrence and spread of the disease is most of all in- 

 fluenced by the humidity of the air and soil, because, in a 

 moist environment, sporangia are abundantly produced on the 

 leaves, and the germination of the sporangia and gonidia, both 

 above and below ground, is greatly favoured. 



If dampness prevail during storage in winter, numerous 

 sporangiophores are produced on the tubers, especially in the 

 region of the eyes, or where a wound may happen to occur ; 

 and by means of the sporangia that are formed the disease may 

 be conveyed to previously sound tubers even at that season of the 

 year. 



PERONOSPORA VITICOLA 



A decade or two ago this parasite of the vine was introduced 

 from America, and in the interval it has rapidly spread through 

 the vineyards of Europe. 



Its American designation of mildew, or grape-vine mildew, 

 has been changed in France to mildiou. In Germany it is called 

 the false mildew of the vine (falscher Mehlthau der Rebeii). 



The disease is characterized by the occurrence of large grey 

 patches on the under side of the leaves, while on the upper side 

 the infested spots become yellow or reddish. The diseased 

 spots dry up, and the leaves are shed prematurely. During 



F 



