Diseases of Plants. 29 



ever, in fine, be the system of nosology to which we are 

 attached, it is impossible for us to put our hand upon any one 

 class or order of diseases which they describe, without putting 

 our hand, at the same time, upon some disease to which plants 

 are subject in common with animals." 



In previous communications upon the food and climate of 

 plants, I have made a general statement of what may be termed 

 their dietetics, in a treatise on" vegetable pathology ; and, as a 

 due attention to those points is the best prevention of then- 

 diseases, I shall confine myself from any general remarks on 

 that point, and to the specific diagnosis, prognosis, and treat- 

 ment of such, disorders as I may mention. 



Canker, Ulcer. These are synonyms of the same disease, 

 which is accompanied with different symptoms, according to 

 the species of tree in which it occurs ; being known as the 

 canker in those whose true sap contains a more than usual 

 proportion of acid ; and as ulcer in those containing tannin, or 

 other astringent principle. In both it is an ulcerous affection. 



The symptoms vary in the genus Pyrus, and others whose 

 sap contains a free acid. It is seldom, if ever, accompanied 

 by a discharge; but the parts affected enlarge, their wood be- 

 comes brown and carious, and the bark covering them cracks, 

 gapes, and manifests a disposition to separate from the wood. 

 In the genera L/'lmus, Quercus, and others abounding in 

 astringent matters, a copious discharge usually accompanies 

 all the preceding symptoms, which are always present also, 

 with the exception of the swelling of the parts, which only 

 occasionally occurs. The discharged liquid is occasionally 

 transparent ; but generally a reddish brown, becoming nearly 

 black by exposure to the air. The sides of the ulcer, in the 

 first instance, are usually covered with a white, crystalline, in- 

 crustation ; in the second, with a shining, varnish-like coat. In 

 the genus Primus, and others abounding with gum, the same 

 symptoms are exhibited, except that swelling still more rarely 

 occurs, and that the discharged matter is nearly pure gum, a 

 variation which seems to remove it to another class of diseases. 

 In every instance, I am prepared to maintain that the disease 

 is local ; that is, it at first arises from a disarrangement in the 

 functions of the affected part, and is never brought on from a 

 general diseased state of the tree ; but is occasioned by contin- 

 gencies perfectly independent of soil and situation. When the 

 disease has commenced, if these are unfavourable, they may 

 aggravate the symptoms, and promote their diffusion, but they 

 are not the originators of the disease. 



It appears to me, in general, to arise from contused wounds, 

 however they may be inflicted ; by the bruise occasioned by 



