xxxv.] POTATO DISEASE, L ITS ACTIVE STATE. 289 



originally attached to the stem of the parent fungus at 

 B. It must be specially noted that water or moist air 

 is essential for the existence of the fungus, for nearly 

 every part speedily perishes in dry air, heat, or frost. 

 When the conidia burst and set free the minute zoospores, 

 the latter sail over the damp surfaces of leaves, and even 

 float into the organs of transpiration. A zoospore swimming 

 in an intercellular space is shown at R, Fig. 127. One 

 has only to imagine a large field of potatoes, with all the 

 leaves moist and swaying backwards and forwards with 

 the wind, to perceive that such a field, say on a warm 

 misty morning or evening, would form a sort of continuous 

 lake of moisture on which the zoospores could float from 

 one plant to another. The conidia, with the contained 

 zoospores, are also carried through the air in millions by 

 the wind ; they are so lightly attached to their sup- 

 porting stems and so extremely small and light, that the 

 faintest breath of air wafts them away. Insects and 

 other creatures also carry the conidia from place to place. 

 The flies which alight on potato plants carry off hundreds 

 of conidia on their bodies. If a bird drops in a field of 

 diseased potatoes, the fluttering of its wings will disperse 

 millions of the conidia of the fungus of the potato murrain 

 into the air. The same phenomenon occurs when a dog 

 or other animal runs amongst diseased potato plants. 

 When the conidia or zoospores burst and germinate, the 

 threads which emerge are corrosive or putrefactive. To 

 such an extent is this the case that the spawn is said to 

 be capable of piercing or boring through the cuticle of 

 the leaf from within or without, regardless of the natural 

 openings or stomata, and even of piercing the bark of the 

 stem or the tuber itself. 



The fungus of the potato disease generally attacks the 

 leaves first, and, as the leaves produce successive crops of 

 fungus growth, the disease quickly spreads to the leaf- 

 stalks, from the leaf-stalks to the chief stems, and from 

 the stems to the tuber. Sometimes a week or two elapses 



