254 PRINCIPLES OF GARDENING. [CH. VIII. 



explains the causes, and throws light upon the pre- 

 vention of many diseases affecting those which we 

 cultivate, and warns the cultivator from the late per- 

 formance of many of his operations, as Avell as from 

 being needlessly violent in his treatment. If a grape- 

 vine be pinmed too late in the spring, the bleeding 

 or effusion of sap has been known to be so excessive, 

 that the tree has died from absolute exhaustion. 

 Stone-fruit trees, if severely bruised, are frequently 

 destroyed by the inroads of a disease, resembhng, in 

 all its characteristics, the cancerous affections of ani- 

 mals ; and I have known a whole crop of wheat af- 

 fected with a swelling of the stem or culm, evidently 

 caused by an extravasation of the sap from its rup- 

 tured vessels, owing to a hea\7- roller being passed 

 over the crop, Avhen of a forward growth. 



I shall now proceed to the consideration of a 

 few of the most usual diseases affecting cultivated 

 plants, but without any attempt at classification, for 

 our knowledge is too imperfect as yet to justify an 

 attempt to form a system of vegetable nosology. It 

 is readily perceivable that plants have their epi- 

 demics, for at certain seasons a disease, such as the 

 mildew, will devastate a whole neighbourhood. They 

 have their endemics for some diseases, as the 

 Amhury is often confined to a single compartment 

 in a garden ; but these and other detached portions 

 of similar knowledge are too slight and uncoimected 



