406 



BOTANY OF THE LIVING PLANT 



not end with the death of its host. There is in fact no absolute line 

 between the parasitic and the saprophytic habit. 



The risk of attack on the seedlings may be reduced by avoiding 

 too close sowing and too moist culture ; also by avoiding the use 

 of soil in which infected seedlings have been raised. The risk may be 

 further reduced by raising the soil before sowing to a temperature 

 that will kill all the germs that it contains. 



The Potato Fungus (Phytophthora infestans). 



The Potato Fungus is the cause of what is commonly known as 

 the " Potato Disease," which is always a risk to the crop in damp, 

 warm seasons. In the years 1 845 -1 850 the disease already known 



Fig. 305. 



A Potato-leaf, showing the spots and patches of " Potato-disease ", due to Phyto- 

 phthora infestans. In the darker patches the tissues are quite dead. The margins 

 of the spots would show the hvphae of the fungus projecting from the surface. (After 

 Sorauer ; from Marshall Ward.) 



in America assumed epidemic virulence in Ireland, causing the great 

 famine. Since that time the potato crop has never been entirely 

 free from it. The disease makes its appearance upon the leaves and 

 stems as spots at first small and pale-coloured, but as they enlarge 

 the centre of each becomes brown, and extends, though still with a 

 pale margin, till the spots run together, and the whole leaf or even the 

 whole shoot may be affected (Fig. 305). If leaves with young infected 

 patches be examined on a damp still day, or better, if they be kept in 

 moist still air under a bell-glass, white glistening filaments will be 

 found on the lower surface. They are large enough to be seen with 

 the naked eye, and are the sporangiophores of the fungus. They bear 



