155 



than by ihe irregular supplies of artificial watering, whicli not infre- 

 quently do more harm than good, by causing the surface to bake, the 

 water not penetrating to the roots below."* The moisure of the soil 

 being, acording to Mr. Thomas, thus preserved in the most even man- 

 ner, the potato would not suffer from any of those sudden changes which 

 so frequently take place in summer. It is no uncommon thing for the 

 potato, after attaining a certain size to have to struggle for a precarious 

 existence for weeks and weeks. The " heavens above being as brass and 

 the earth as iron." Nature in this case husbands all her resources, no 

 more branches or twigs are thrust out, the sap-vessels become contracted, 

 the stems shrink, and vegetation all but ceases. The long looked for rain ap- 

 proaches — it descends in torrents — the earth is saturated, and vegetation 

 "drinketh up water like the thirsty ox." They imbibe to repletion. Such is 

 the case with the potato. The sap-vessels are now dilated — the soil, thor- 

 oughly saturated, sends forth noxious gases ; if the weather continues 

 cloudy and no wind is stirring, evaporation is suppressed and the potato 

 dies. Or it may be that then is the " bright shining after the rain." 

 In this case according to Liebig, "When the plant has taken up a max- 

 imum of moisture and the evaporation is suppressed by a low tempera- 

 ture or by continued hot weather, the supply of food, the nutrition of 

 the plant ceases ; the juices stagnate and are altered ; they now pass in- 

 to a state in which they become a fertile soil for microscopic plants. 

 When rain falls after hot weather, and is followed by gi-eat heat without 

 wind, so that every part of the plant is sun-ounded by an atmosphere 

 saturated with moisture, the cooling due to further evaporation ceases, and 

 the plants are destroyed by fire-blasts or scorching (sonnenbrand, Ger- 

 man,) literally, sun-burn or sun-blight." 



As fomierly hinted, the disease may be induced by repletion, after a 

 superabundant fall of rain. A bright gleam of sunshine expanding the 

 stagnated sap, as formerly explained, may cause a rupture in the sap- 

 veesels — a kind of vegetable apoplexy ensues, and the plant suddenly 

 dies.f Besides, it is well known to Agricultural Chemists and farmers 

 acquainted with scientific agriculture, that an excess of carbonic acid 



'Extracted from au excellent article by Mr. Thomas, Patent Office Report (AgricuItivrrJ, - 

 for 1850—51. 



fMinute Animalculae in such cases take possession of the plants, which have led some to 

 the conclusion that they are the cause of the disease. This is mistalving the cause for the ef- 

 fect. It would be as conclusive reasoning to say, on beholding the carcass of an aniraal erawl- 

 ing with maggots, that said maggots were the cause of the animal's death ! 



