262 



point, Mr. Berkeley, in his elaborate treatise published in the London 

 Horticultural Society's Transactions for January, 1846, said in language 

 as plain and decisive as it is possible for language to be, "The decay is 

 the consequence of the presence of the mould, and not the mould of the 

 decay. It is not the habit of the allied species to prey on decaying or 

 decayed matter, but to produce decay ; a fact which is of the first impor- 

 tance. Though so many other species have this habit, these have not.'* 

 Again, " I do not know of any single instance in which any of the nearly 

 allied species have been found in any other situation than growing from 

 the tissues of plants. Were this ever the case, they could not have been 

 overlooked, as their pores are so much larger than any other species of 

 the genus. The species are, in fact, as peculiar to the living tissues of 

 plants as are the several species of Puccinia and Uredo, which could not 

 exist, or at any rate be perfected, elsewhere. 



My task now is to explain, if possible, why the disease is certainly of 

 modern origin, and what causes have come into operation favorable to 

 the increase of the parasite to the present alarming degree. 



Different plants, as well as different animals, have their peculiar para- 

 sites. Some parasitical fungi will indeed prey upon many difterent 

 plants, but the attack of a species is generally confined to a certain natu- 

 ral order or family of plants, or to a genus, or to two or three species of 

 a genus, whilst some, as with parasites in the animal kingdom, exist on a 

 particular part only of one species. The parasitical fungus which attacks 

 and mildews wheat in unfavorable situations or seasons will not live upon 

 turnips. That which infests the turnip will seize upon the cabbage, they 

 being nearly allied plants ; but it will not touch the potatoe, yet the para- 

 sites of the turnip and the potatoe are nearly allied. 



For a plant to be affected by mildew two things then are requisite : 

 the presence of its peculiar parasite and the conditions favorable to the 

 growth of fungi. 



As the potatoe is an exotic, is it not probable that tubers of the plant 

 may have been originally introduced into Europe without its parasite. 

 In the same way it may have been carried by Europeans, to other coun- 

 tries, of which it is not a native, and have, consequently, until lately, 

 remained free from the parasite in those countries also. 



M. Boussingault stated to the Academy of Sciences of Paris, on the 

 authority of M. Joachim Acosta, that the malady is well known, in rainy 

 years, at Bogota, where the Indians live almost entirely on potatoes 



