SECT. IV. ELABORATION OF THE SAP. 143 



juices as may have been absorbed, or secretions as 

 may have been formed, beyond what are necessary 

 to the due nourishment or composition of the plant, 

 or beyond what the plant is capable of assimilating 

 at the time. Hence the watery exudation is per- 

 haps nothing more than a redundancy of the fluid 

 thrown off by imperceptible perspiration,, and the 

 waxy and resinous exudations nothing more than 

 a redundancy of secreted juices ; all which may be 

 still perfectly consistent with a healthy state of the 

 plant. But there are cases in which the exudation 

 is to be regarded as an indication of disease, par- 

 ticularly in that of the exudation known by the 

 name of Honey-dew, a sweet and viscid substance 

 covering the leaves like a varnish, and sometimes 

 occasioning their decay, Such at least seems to be 

 the fact with regard to the honey-dew of the Hop, 

 which, according to the observations of Linnaeus, 

 is the consequence of the attacks of the caterpillar 

 of the ghost moth injuring the root. And such 

 seems also to be the fact with regard to the honey- 

 dew of the Beech-tree, which Dr. Smith regards 

 as the consequence of an unfavourable wind.* But 

 whether the honey-dew of the Oak is to be re- 

 garded as an indication of disease I cannot say, as 

 I have often met with it on trees and leaves that 

 seemed perfectly healthy. The sap then in the 

 progress of its ascent from the extremity of the 

 root to the extremity of the leaf undergoes a con- 

 * Introduction, p. 18y. 



