CH. VIII.] THE DISEASES OF PLANTS. 281 



favourable circumstances under which it is vege- 

 tating, the swelling is greatly increased in size, and 

 the albumiun has become extensively dead ; the su- 

 perincumbent bark cracks, rises ia discoloured scales, 

 and decays even more rapidly than the wood beneath. 

 If the caries is upon a moderately-sized branch, the 

 decay soon completely encircles it, extending through 

 the whole alburnum and bark. The circulation of 

 the sap being thus entu'ely prevented, all the parts 

 above the disease necessarily perish. In the apple 

 and the pear the disease is accompanied by scarcely 

 any discharge ; but in the elm this is veiy abundant. 

 The only chemists who have examined these morbid 

 products are Sir H. Davy and Vauquelin ; the former s 

 obseiTations being confined to the fact, that he often 

 fomid carbonate of lime on the edges of the canker 

 in apple-trees^. 



Vauquelin has examined the sanies discharged 

 from the canker of an elm with much more precision. 

 He found this liquor nearly as transparent as water, 

 sometimes slightly coloured, at other times a blackish 

 bro^vn, but always tasting acrid and saline. From 

 this liquor a soft matter, insoluble in water, is de- 

 posited upon the sides of the ulcer. The bark over 

 which the transparent sanies flows attains the ap- 

 pearance of chalk, becoming white, friable, cr}'stal- 

 line, alkahne, and effervescent mth acids. A 



* Elements of Agric. Chemistry, 2nd ed. p. 246. 



