PHYCOMYCETES. (a) OOMYCETES 417 



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. 352). 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 



FIG. 352. 



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'hyphae of the fungus projecting from the surface. (After 

 Sorauer ; from Marshall Ward.) 



filaments bearing white powdery conidia will be found on the lower 

 surface. They are large enough to be seen with the naked eye, and 

 are the conidiophores of the fungus. They spring from the mycelium, 

 which permeates the mesophyll of the leaf in the diseased patch 

 (Fig- 353)- The mycelium consists of coarse non-septate, branched 

 hyphae, which traverse the intercellular spaces, coming into close 

 contact with the moist walls of the cells. They are also able to 

 penetrate the softer middle-lamella of the cell-walls, where two cells 

 adjoin, and this brings them into still closer relation to the cells, 

 and the nourishment which these can supply (Fig. 354). Though the 

 cells are not as a rule perforated, they lose their vitality and collapse, 

 probably owing to a toxic influence. The brown discolouration at 

 the centre of the infected spots is due to their decay. 



B.B. 21J 



