182 OBSERVATIONS ON THE DISEASES 01' PLANTS 



A marked excess of pliosphorated lime and magnesia is exlii- 

 bited here above sidphated and nmriated alkalies. 



4. This is not the place to treat on every individual malady to 

 which plants are subject, but merely to bring forward the general 

 points of view whicli arise from a physiologico-chemical exami- 

 nation of the little which we know of the diseases of plants. It 

 is not my intention to advert to the destruction of plants or por- 

 tions of plants arising palpably from external agents, but merely 

 to examine the alterations which depend upon an inward abnormal 

 condition. 



Two things must here be distinguished, the diseases of plants 

 in free nature and those under cultivation, including those wild 

 species on which it exercises an indubitable influence, as corn- 

 weeds. As regards the first, the question may be asked, whether 

 diseases at all affect them, or are capable of doing so, when they 

 are left entirely to nature, and are removed from every influence 

 of man ? The answer to this question would lead us too far. 

 It is, however, indubitable that diseases do arise in apparently 

 wild plants, as for instance tlie Tyrolese fir-woods were very 

 generally visited by Rust in 1845. The causes of such diseases, 

 however, may perhaps be completely external and merely tran- 

 sitory, and rest most probably on tlie peculiar atmospheric con- 

 ditions of particular years, and therefore vanish of themselves in 

 the succeeding season. 



But in cultivated plants it is entirely different. Almost all of 

 them are no longer in their natural condition, but, regarded in 

 a scientific point of view, are in themselves deviations from their 

 normal condition. The natural equilibrium in their formation 

 is abolished by the unnatural and excessive development of 

 especial circumstances of form or of particular elements, and they 

 present, to the external injurious influences which in part subsist 

 in the very modes of cultivation, a greater number of weak 

 points. To this is added that a great part of the objects of cul- 

 ture are not in their natural climate, but in a zone whicli is 

 strange and ungenial, and that consequently they are affected by 

 a number of inevitable causes of disease.* It scarcely requires 



* This mode of viewiug the case leads to the comfortless conclusion, 

 that we shall perhaps never be in a condition to extirpate completely the 

 diseases of our cultivated plants, and that as with us people reckon on the 

 tenth grain, but not upon the hundredth, as is the case in Chili, or as in 

 some places in every great calculation a certain per centage is deducted for 

 the probable destruction of hailstones, so it would be better, instead of 

 writing thick volumes and even little libraries containing treatises on the 

 occurrence and treatment of diseases, to collect correct statical data, with 

 a view to calculate the mean loss due to disease, so as to make it harmless 

 by reason of foresight. In a political point of view beyond doubt the firm 

 establishment of such statical conditions is of the greatest importance, for in 



