282 DISEASES OF FIELD & GARDEN CROPS. [OH. 



one advanced by Mr. Baker, and say it is impossible to 

 over-cultivate any plant ; that seedling potatoes are as 

 badly affected as those grown from cut tubers ; and that 

 animals and plants placed in an artificial position by man 

 only need an extra amount of care corresponding with 

 their new position. Race horses are said to live as long 

 as cart horses, domestic as long as wild animals, and the 

 delicate children of towns as long as the more roughly 

 nurtured children of country villages. It is acknowledged 

 that extra care is required, but it is maintained that the 

 constitution is not impaired. 



In the potato disease, as in every other disease, both of 

 animals and plants, it is necessary that the ailment should 

 be completely understood before any serious attempt can 

 be made towards the prevention or palliation of the 

 attacks. We will, therefore, closely examine the struc- 

 ture and mode of growth of the potato fungus. 



For an exact examination of Peronospora infestans, 

 Mont., a very minute and extremely thin and transparent 

 slice must be cut from a diseased leaf at a spot where 

 the white bloom caused by the presence of the fungus is 

 visible underneath. A good plan is to cut a diseased 

 leaf in two through a disease spot, and then with a sharp 

 lancet cut an extremely thin slice off from one of the ex- 

 posed cut surfaces. If the slice last cut is somewhat 

 longitudinally wedge-shaped, it will often best show the 

 structure of the leaf and the contained fungus at the 

 thinner end of the section. Such slicing requires great 

 care and experience, and the art is only acquired after 

 many failures. To those, therefore, who are unequal to 

 the task we advise the purchase of slides ready prepared 

 by the Rev. J. E. Vize. The atom to be examined 

 should be placed on a glass slide in a drop of glycerine 

 (this is preferable to water, as the latter often dries too 

 quickly), and then covered with a clean thin cover-glass. 



The magnification given by an ordinary lens is useless for 

 the observation of the minute fungus now before us, so we 



