WARTS ON THE BACK OF THE LEAF. 59 



a climate as the mildew will grow in. It is not uncom- 

 mon for individuals to have some favourite day in June, 

 perhaps, when they give up firing their vinery. They 

 still continue the usual sprinkling with water, and shut 

 up the house with a stagnant atmosphere, loaded with 

 moisture ; and it often happens that cold nights reduce 

 the temperature of the vinery so low that the vital 

 energy of the vines is depressed, predisposing them to 

 disease, while they are in a climate well adapted to the 

 growth of fungi of any sort. To a careful attention to 

 the keeping up the proper degree of heat during the 

 whole forcing season, not too much moisture, and a 

 constant circulation of fresh air, I attribute the exemp- 

 tion from vine mildew I have experienced, when 

 vineries not five hundred yards off had their crops 

 ruined by it. It has been supposed by some that the 

 mildew merely makes its appearance as the conse- 

 quence of a diseased condition of the tissues of the 

 vine ; but this I hold to be a mistake. The spores 

 of the parasite in question may exist in myriads on 

 every inch of the vine's surface and do it no harm, 

 unless the climate of the vinery is made to suit their 

 development, when they spring into life as if by magic, 

 and arrest the growth of all they attack. In a hot and 

 rather dry climate they never can do this, and in such 

 the vine may be considered safe from their effects. 



WARTS ON THE BACK OF THE LEAF. 



.;;.; . . .' , ' 



This is a sort of conglomerate of little green warts 

 that form on the lower surface of the leaf, as if the re- 

 sult of an extravasation of sap through its epidermis or 



