CH. VIII.] TnE DISEASES OF PLANTS. 299 



of the sap effused. 3. From tlie unnatural contrac- 

 tion of the circulatory vessels. 4. From wounds. 

 5. Heat and dryness. 



1. With regard to the alkaline state of the sap, 

 it may be observed, additionally, that the excessive 

 alkaline quality of the sap, imparting to it the power 

 of destroying the fibre of its containing vessels, is 

 placed on the basis of chemical experiment. A weak 

 alkalme solution dissolves woody fibre, Avithout altera- 

 tion ; and it may be thrown down again by means of 

 an acid. By this property we are enabled to sepa- 

 rate wood from most of the other vegetable princi- 

 ples, as few of them are soluble in weak alkaline leys^. 

 It is tiTie that the vital principle may counteract 

 powerfully this chemical action ; but it will not con- 

 trol the corrosive effect of an active agent in excess, 

 if repeated for any length of time. The blood of the 

 human system contains, when in a healthy state, a 

 portion of common salt ; yet if this saline constituent 

 is in excess, it induces inflammation and organic de- 

 rangement. 



'2. Plethora is that state of a plant's excessive 

 vigour, in which the sap is formed more rapidly than 

 the circulatory vessels can convey it away. When 

 this occurs, inipture must take place ; for the force 

 with which it is propelled during circulation, and 

 cons^equently, the force acting to burst the vessels 



^ Thomson's System of Chemistry, vol. iv. p. 180. 6th ed. 



