CH. VIII.] THE DISEASES OF PLANTS. 305 



the fii-st and third named causes is probable, or 

 rather certain, but this is by no means conclusive 

 that there is not a similar liquid extravasated upon 

 the surface of the leaves, o^^ing to some unhealthy 

 action of their vessels. It is vrith this description of 

 honey.dew that we are here concerned. The error 

 into which writers on this subject appear to have 

 fallen, consists in then* having endeavoui'ed to assign 

 the origin of every kind of honey-dew to the same 

 cause. Thus the Rev. Gilbert White seems (Natu- 

 ralist's Calendar, 144) to have had a fanciful and 

 comprehensive mode of accountuig for the origin of 

 honey-dew: he tells us, "June 4, 1783. Vast honey- 

 dews this week. The reason of this seems to be, 

 that in hot days the effluvia of flowers are dmvm. up 

 by a brisk evaporation, and then in the night fall 

 down with the dews with which they are entangled." 



The objection ui'ged to this theoiy by Ciu'tis 

 (Trans. Linn. Soc. vi. 82) is conclusive. " If it fell 

 from the atmosphere, it would cover every thing on 

 which it fell indiscriminately, whereas we never find it 

 but on certain liviug plants and trees ; we find it also on 

 plants in stoves and green-houses covered -svith^lass." 



Curtis had convinced himself that the honey-dew 

 was merely the excrement of the aphides ; and he 

 supported his theoiy Ts-ith his usual ability, although 

 he justly deemed it a little " wonderful extraordin- 

 ary," that any insect should secrete, as excrementi- 



X 



