ORIGIN OF THE POTATO BLIGHT. 87 



taken up a maximum of moisture, and the evapora- 

 tion is suppressed by a low temperature or by con- 

 tinued wet weather, the supply of food, the nutri- 

 tion of the plant, ceases ; the juices stagnate, and 

 are altered ; they now pass into a state in which 

 they become a fertile soil for microscopic plants. 

 When rain falls after hot weather, and is followed 

 by great heat without wind, so that every part of 

 the plant is surrounded by an atmosphere saturated 

 with moisture, the cooling due to further evapora- 

 tion ceases, and the plants are destroyed by fire- 

 blast or scorching (Sonnenbrand, German, literally 

 sun-burn or sun-blight). 



After the experience and observations of so long The potato 

 a period in reference to the influence of evaporation p robabiy S a 



on the condition of plants, I hardly think that any 

 unprejudiced observer can entertain the smallest 

 doubt concerning the cause of the great mischief 

 which has befallen agriculture during the last few 

 years. If HALES, that unequalled observer and 

 inquirer, had known the potato disease, I hardly 

 believe that he would have ascribed it to an internal 

 cause belonging to the plant, any more than he 

 thought of ascribing the blight of the hop-plants, 

 formerly mentioned, to a special hop disease, or the 

 rotting of the apples to an apple disease. Even The potato 

 PARMENTIER, to whom France is indebted for the 



introduction of the potato, knew this disease, and 

 has very accurately described it. The term " po- 

 tato-rot" has been known to the oldest peasants 



