304 PRINCIPLES OF GARDENING. [CH. VIII. 



essential difference, that in the case of the plants 

 the extravasation is upon the surface of the leaves, 

 and consequently, in proportion to the abundance of 

 the extruded sap is their respiration and digestion 

 impau'ed. 



Azaleas sometimes, hut rarely, have the impubes- 

 cence on their leaves, especially on their lower sur- 

 face, beaded as it were with a resinous exudation. 

 This can scarcely he called a disease. It is never 

 found but upon plants that have been ke2)t in a 

 temperatiu'e too high, and in a soil too fertile. It is 

 an effort to relieve the surcharged vessels, and occurs 

 in various forms in other plants. 



The honey-dew was noticed by the ancients and is 

 mentioned by Pliny ^ by the fanciful designation of 

 the " sweat of the heavens," and the '• saliva of the 

 stars," though he questioned whether it is a deposition 

 from the air, purging it from some contracted im- 

 purity. More modem philosophers have been quite 

 as erroneous and discordant in their opinions relative 

 to the diseases nature. Some, with the mostunmi- 

 tigatable asperity declare that it is the excrement of 

 aphides : others as exclusively maintain that it is an 

 atmospheric deposit ; and a third party consider that 

 it arises from bleeding consequent to the woimds of 

 insects. That there may be a glutinous, saccharine 

 liquid found upon the leaves of plants arising from 



^ Hist. Natur. xi. 12. 



