POTATO DISEASE. 97 



which within 15 to 18 hours, developes an abundant crop of spores. 

 From this insignificant beginning may spring the most destructive 

 results, as will be plain when we consider the number of spores 

 which are produced, the ease with which they are detached from 

 the spore sacks, the fact that they retain their vitality for several 

 weeks, and the extraordinary rapidity with which they reproduce 

 new generations of fungi. 



The rapidity with which the Perononpora infestans propagates 

 from the mycelium contained in seed potatoes, is at first slow, and 

 for a time proceeds, as has been remarked, without perceptibly 

 injuring the vigor or luxuriance of the stems and foliage of the 

 potato plant. This statement is not a mere inference from what is 

 known as to the potato disease, but is proved by actual experiment. 

 Be Bary infected healthy potato plants having vigorous foliage, 

 wijth fungus spores, in a room where the uniform condition of the 

 atmosphere was certainly far more favorable to fungus development 

 than the free air usually is, and he found that where hundreds of 

 germs had penetrated the potato stems, it required 29 days befo-rc 

 the fungus had spread through 8 inches of stem in one case, and', 

 through 4 inches in another. In these instances fructification did-' 

 not take place, and the potato plants grew well, branching and'i 

 leafing out luxuriantly. 



If the fungus is sown upon healthy potato leaves, they often- 

 remain to all appearance healthy for a long time, even when micro- 

 scopic investigation demonstrates that the fungus has penetrated 

 the tissues. 



These facts explain why the disease does not at once ravage a 

 field into which it has been introduced by the planting, but on the 

 contrary remains comparatively dormant until the potato has 

 attained its full development, and the time of the year arrives when 

 the external conditions are most suitable for a rapid and devastat- 

 ing growth of the fungus. 



It is easy to imitate artificially what thus happens in nature, and 

 at any season to change the slow process of infection to the rapid 

 one of destruction. De Bary made the following green-house ex- 

 periment : In February three vigorous potato stocks grown in 

 pots were placed in the immediate vicinity of some artificially 

 infected shoots, on which Peronospora existed in a state of fructi- 

 fication. The plants were now frequently watered, the foliage 

 being copiously besprinkled. In a short time the fungus estab- 

 7 



