40 MOTION OF THE JUICES OF THE ANIMAL BODY. 



temperature and dryness of the atmosphere. The absorbent power of plants the 

 motion of their sap, depends on evaporation ; the amount of food necessary for 

 their nutrition, which is absorbed, is proportional to the amount of moisture given 

 out (evaporated) in a given time. When the plant has taken up a maximum of 

 moisture, and tha evaporation is suppressed by a low temperature or by continued 

 wet weather, the supply of food, the nutrition 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 fol- 

 lowed 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 evaporation 

 ceases, and the plants are destroyed by fire-blast or schorching (Sonnenbrand, 

 German, literally, sun-burn or sun-blight.) 



After the experience and observations of so long a period in reference to the 

 influence of evaporation on the condition of plants, I hardly think that any un- 

 prejudiced 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 men- 

 tioned, to a special hop di-sease, or the rotting of the apples to an apple disease. 

 Even PARMENTIER, to whom France is indebted for the introduction of the potato, 

 knew this disease, and has very accurately described it.t The term " potato-rot " 

 has been known to the oldest peasants and agriculturists since their youth ; it has, 

 doubtless, only acquired of late years the frightful significance, which seems to 

 threaten the well being of nations, since the causes, which formerly brought it 

 locally into existence, have spread over whole districts and countries. The writings 

 of HALES bring to our century from a preceding one the consoling certainty (and 

 this is especially important,) that the cause of this decay is not to be looked for in 

 a degeneration of the plant, but depends on the combination of certain conditions 

 accidentally coincident ; and that these, when they are well ascertained and kept 

 in view, enable the agriculturist, if not to annihilate, at least to diminish, their hurt- 

 ful influence.! 



The potato plant obviously belongs to the same class of plants as the hop plant, 

 namely, to that class which is most seriously injured by the stagnation of their 

 juices in consequence of suppressed transpiration. According to KNIGHT, the 

 tubers are not formed by swelling of the proper roots, but by the development of a 

 kind of underground stalks or runners. He found that when the tubers under 

 ground were suppressed, tubers were formed on the stalks above ground ; and it is 

 conceivable that every external cause which exerts a hurtful influence on the 

 healthy condition of the leaves and stalks, must act in like manner on the tubers. 

 In the districts which were most severely visited by the so-called potato disease in 

 1846, damp, cold, rainy weather followed a series of very hot days ; and in 1847, 

 cold and rain came on, after continued drought, in the beginning of September, 

 exactly at the period of the most luxuriant growth of the potatoes. || 



In most places, no trace of disease was observed in the early potatoes before the 

 middle of August ; and even after that period low-lying, cold and wet fields, were 

 chiefly attacked by it. In many plants, in the same field, in which the seed pota- 

 toes had been destroyed by putrefaction and decay, the tubers appeared quite healthy, 

 while in others it was easy to see that these tubers alone, which lay next to the 

 old potatoes, were infected and attacked by the disease, and that on the side next 

 to the old tubers. ^f i 



In 1846 all the potato plants in my garden died completely off towards the end 

 of August, before a single tuber had been formed ; and in 1847, in the same field, 



* The potato blight has probably a similar origin. 



t The potato blight has been long known. 



t It is not due to a degeneration of the plant, but to a combination of external causes. 



$ The potato plant is one of those which suffers most from suppressed evaporation. 



H Character of the weather in 184G and 1847, when the potato blight prevailed. 



if In most places the early potatoes escaped till after the middle of August. 



