OOMYCETES. 



105 



appear to the naked eye as a fine mould on the surface of the 

 plant. The disease soon makes itself known by the brown colour- 

 ing of those parts of the plant which are attacked, and by their 

 withering. An ovoid conidium arises at first by the formation of 

 a dividing wall at the apex of each branch of the conidiophove, 



FIG. 89. Peronospora. alsinearum. My 

 celium with egg-cell and antheridium. 



FIG. 90. Ph\jtoplithora infestans (strongly magnified). Cross section through a small 

 portion of a Potato-leaf (the under side turned upwards): a the mycelium ; b b two conidio- 

 phores projecting through a stoma; c conidia; e the spongy tissue of the leaf; g the 

 epidermis. 



