190 OBSERVATIOXS ON THE DISEASES OF I'LANTS 



same plant, sound and highly-diseased tubers occur close to each 

 other ; for it scarcely requires explanation that the soil of the 

 same inclosure is by no means a uniform admixture of tiie sub- 

 stances which it contains. The luxuriant vegetation and deep 

 green in certain spots points out in the second year the places 

 where the mannre heaps were laid, and from whence they were 

 distributed over the surface. Every decided difference of soil, 

 however local, must necessarily induce a difference of vegetation, 

 and especially of the chemical ])rocess in the plant, so that 

 individual tubers may be exposed to injurious action in very 

 different degrees. 



6. One of the most practically important questions which now 

 arise is, whether the disease which has prevailed since 1845 is to 

 be regarded as an isolated fact, as a circumstance induced by a 

 rare and altogether peculiar concurrence of circumstances, or 

 whether in future similar dangers threaten us. The answer to 

 tins question will naturally be very different, according to the 

 views which are entertained of the malady. Those which have 

 at present been put forth may be arranged under three heads : — 

 1. It is attributable either to vegetable or animal parasites. 

 This merits no further notice, for it can escape only the most 

 superficial observation that the diseased symptoms occur, witli- 

 out exception, before the least trace of the parasites is visible, 

 and the most numerous voices, and those of most weight in 

 science, have been raised against it. 2. Others attributed the 

 disease to the peculiar weather of 1845, and prophesied its total 

 disappearance in more favourable years. Painful experience has 

 disproved any such opinion, but unfortunately observation has 

 in consequence been diverted into a wrong channel. 3. The 

 right view, and that which has most promise for the future, 

 springs from the history of the potato. It thence appears that 

 the malady is no isolated fact, but that diseased action has from 

 the earliest records increased in extent and intensity. 



Potatoes, with rare exception, have been grown in freshly 

 manured soil. In all schemes of rotation our best authorities 

 commence with the potato, and some direct the set to be placed 

 immediately on the manure. Putsche and Vertuch, in their 

 monograph, declare that the ground cannot be over-manured. 

 These views have continually been more firmly held, in pro- 

 portion as the cultivation of the potato has extended. In a few 

 parts only of Germany, especially in Thuringia, as above- 

 mentioned, a custom has been pretty well established not to 

 plant potatoes in fresh manure, but as the third or fourth crop, 

 and generally after clover, and these parts are precisely those 

 which remained the longest free from disease, and which suffered 

 the least. 



