CH. VIII.] THE DISEASES OF PLANTS. 279 



are wanting, or if improper components are intro- 

 duced, disease is the necessaiy consequence. Dis- 

 ease, which in youth and manhood usually arises 

 from intemperance and over-excitement, visits old 

 age as a consequence of its decayed \dtal powers; 

 and, " if the silver cord has not been loosed," or 

 " tlie golden bowl broken" by the short-sighted indul- 

 gence of early years, man gradually declines into the 

 ■ grave, as the vital organs cease to perfonn their of- 

 fices, because the limit of existence natui'al to his 

 species has been attained. Some diseases pecidiar 

 to old age are prematurely induced in the usually 

 vigorous period of life by licentious indulgences, in- 

 di\idual or hereditaiy. Ossification of the vascular 

 system is an example. In the vegetable part of the 

 creation, the canker or ulcer, to which our apple, 

 pear, elm, and other trees are subject, is a somewhat 

 parallel instance. This disease is accompanied by 

 different symptoms, according to the species of the 

 tree which it infects. In some of those whose tme 

 sap contains a considerable quantity of free acid, as in 

 the genus Pyms, it is rarely accompanied by any dis- 

 charge. To this dry form of the disease it would be 

 well to confine the term canker, and to give it the 

 scientific name of Gangrana sicca. In other trees, 

 whose sap is characterized by abounding in astringent 

 or mucilaginous constituents, it is usually attended 

 by a sanious discharge. In such instances, it might 



