332 
ROSE LOUSE. 
discriminately ; whereas we never find it but on 
certain living plants and trees. We find it also on 
plants in stoves and green-houses covered with 
glass. If it exsuded from the plant, it would ap- 
pear on all the leaves generally and uniformly : 
whereas its appearance is extremely irregular, not 
alike on any two leaves of the same tree or plant, 
some having none of it, and others being covered 
with it but partially. But the phenomena of the 
honey-dew, with all their variations, are easily ac- 
counted for by considering the aphides as the au- 
thors of it ; that they are capable of producing an 
appearance extremely similar to that of honey-dew 
has already been shown. As far as my observation 
has - extended, there never exists any honey-dew 
but where there are aphides ; such, however, often 
pass unnoticed, being hid on the under side of the 
leaf. Wherever honey-dew is observable about a 
leaf, aphides will be found on the under side of the 
leaf or leaves immediately above it, and under no 
other circumstances whatever. If by accident any 
thing should intervene between the aphides and 
the leaf next between them, there will be no ho- 
ney-dew on that leaf. Thus then we flatter our- 
selves to have incontrovertibly proved that the 
aphides are the true and only source of the honey- 
dew.” 
The sooty appearance which we occasionally find 
on the surface of the bark and foliage of differ- 
ent trees is pwing entirely to the saccharine sub- 
stance which the aphides deposit, and which, after 
