Ill 



CHAPTER XX. 



THEORY OF THE DISEASE. 



Resume of the various supposed causes of disease (439, 440). 

 Vastator (441). The disease (443). Condition of plant causing 

 death (444, 445). Effect of disease on wild plants (446). On 

 cultivated plants (447). Propagation from diseased sets (448, 

 449). As to cessation of disease (450). Relation of health of 

 plant to the insect (451, 452). 



(439.) WE have now examined all the various effects 

 which may influence the potatoe plant in its growth, and 

 give rise to the disease. In the course of our investiga- 

 tions we have found that the action of temperature, light, 

 soils, and manures, may influence the plant, but positive 

 proof has been afforded that they do not produce the pre- 

 sent alarming epidemic. 



(440.) It appears, moreover, that vegetable parasites 

 are sometimes present ; but, upon an attentive examina- 

 tion, there are strong reasons to believe that they occur 

 after the disease has been set in action, and that they grow 

 in the decaying matter, as a beautiful provision against 

 offensive putridity. 



(441.) The case, however, is quite different with the 

 animal parasites ; for we have discovered that wherever 

 the Aphis vastator exists, there the leaves perish, there the 

 plant becomes injured, there its tubers and stems take on 

 gangrene and die, and thus the disease is manifested. 



(442.) In this way we have learnt the cause of the 

 disease, and the theory of the disease is easily ascertained 

 from a study of its cause. 



