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other fungi have been found growing within the stem and tubers. 

 We may therefore fairly say, that the development of fungi has a 

 striking connexion with the premature decay of the tubers. But the 

 question may be asked, Is this development of fungi the cause or the 

 consequence of the disease ? It requires for the elucidation of this 

 point, that the general history and physiological relations of epiphytic 

 fungi should be brought under review, and to this task I shall first 

 proceed. We may for the present purpose divide the whole of the 

 fungi into two classes, those that grow on dead animal or vegetable 

 matter, and those which infest living plants. Of the former, those 

 which grow on the common soil, generally spring up where this has 

 been well manured by the decay of animal manure, of animals them- 

 selves, or on stumps of trees and other decaying vegetable substances. 

 The latter consisting chiefly of the ordinary diseases of corn and other 

 plants, belonging to the genera Uredo, ^Ecidium, Puccinia, Aregma, 

 &c., are as evidently absorbed by the root, carried up by the stem, and 

 finally develope themselves in various parts of the plants, according to 

 their peculiar nature. These plants consist of simple cells contain- 

 ing spores, and, like the parasitical animals, may exist in the plant with- 

 out causing much injury, except to the particular part which they infest, 

 unless the parasite is developed in great abundance. Neither does 

 the fungus cause the destruction of the vitality of the cells of the stem, 

 or of the leaves, except those immediately occupied by it. Other 

 genera of fungi, belonging to a different order, fix themselves on the 

 leaves of plants, and are probably produced in the same way as those 

 just noticed, namely, by the absorption of the spores by the roots, and 

 their transit with the sap to the leaves, where they are developed. 

 The genus Dothidea is an example, Asteroraa another, but still these, 

 although causing some unhealthy appearance in the leaves, do not 

 cause that absolute death of the cells which is seen in the patches of 

 the leaves in the potato disease. 



Descending to the tribe Mucedines, we find that very few of these 

 grow on living plants, and even when such is the case, they spring 

 from previously diseased spots of the leaves ; these spots become yel- 

 low, the cellules flaccid, and in a great measure deprived of vitality, 

 before they afibrd a fit nidus for the development of the fungus. 

 In the genus Botrytis, of which the fungus of the potato is a species, 

 we have three species inhabiting still living but unhealthy plants, 

 and this previous state of debility or diminution of vitality of the 

 diseased plant, is especially observable in one of these cases. It is 

 common to find the Shepherd's purse {Thlaspi Bursa-pastoris) covered 



