SECT. iv. JELABORATION OF THE SAP. 141 



goes also a further modification in consequence of 

 what may be called perceptible perspiration, which 

 is an exudation of sap too gross or too abundant 

 to be dissipated immediately, and which hence ac- 

 cumulates on the surface of the leaf. It is very 

 generally to be met with in the course of the 

 summer on the leaves of the Maple, Poplar, and 

 Lime-tree ; but particularly on the surface exposed 

 to the sun, which it sometimes wholly covers. Its 

 physical as well as chemical qualities are very dif- 

 ferent in different species of plants ; so that it is 

 not always merely an exudation of sap, but of 

 sap in a high state of elaboration, or mingled 

 with the peculiar juices or secretions of the plant. 



Sometimes it is a clear and watery fluid conglo- 

 merating into large drops, such as are said to have 

 been observed by Mr. Millar, of Chelsea, exuding 

 from the leaves of the Musa arbor> or Plantain- 

 tree ; and such as are sometimes to be seen in hot 

 and calm weather, exuding from the leaves of the 

 Poplar, or Willow, and trickling down in such 

 abundance as to resemble a slight shower. This 

 Phenomenon was observed by Dr. Smith under a 

 grove of Willows, in Italy,* and is said to occur 

 sometimes even in England.-}- Sometimes it is 

 glutinous, as on the leaf of the Lime-tree ; some- 

 times it is waxy, as on the leaves of Rosemary; 

 sometimes it is saccharine, as on the Orange leaf, 

 according to the account of M. De la Hire, as re- 

 * Lectures at R. lust. + Introd. p. 188. 



