86 AUSTRALASIAN 



and is sweet like honey ; " while some assert that it is the 

 secretion of aphides, or plant lice ; and others, that it is simply 

 an exudation from the plants or trees on which it is found. 

 The first-mentioned idea is now quite exploded ; the two latter, 

 between them, contain the truth. German apiarists make a 

 distinction between Honigthau (honey-dew), which they look on 

 as an excretion of aphides, and Nebenblatthonig, which they 

 consider to be exuded by the leaf-buds of certain plants or 

 trees. Liebig asserts that when 



" the quantity of sugar present (in the sap of a tree) is greater 

 than can be exhausted by the leaves and buds, it is execeted from the 

 surface of the leaves or bark. Certain diseases of trees, for example 

 that called honey-dew, evidently depend on the want of the due 

 proportion between the quantity of azotised and that of the una7,otised 



substances which are applied to them as nutriment When a 



sufficient quantity of nitrogen is not present to aid in the assimilation 

 of the substances destitute of it, these substances will be separated 

 as excrements from the bark, roots, leaves, and branches. The exuda- 

 tions of gum, mannite, and sugar, in strong and healthy plants, 

 cannot be ascribed to any other cause." 



He instances several cases of copious exudation of a thick 

 sweet liquid from the leaves of trees, more especially from 

 those of the linden, during a hot and dry summer. Ludwig 

 Huber, in his fflenenzucht (1880), mentions three sorts of 

 Honigthau, one which is voided by the plant lice, which is the 

 worst ; " it is tough, tasteless, and unhealthy for the bees in 

 winter ;" another is obtained from the leaves of the oak, which, 

 when bitten by a certain kind of beetle, give out, " by favour- 

 able temperature, a sweet liquid, which the bees eagerly lick 

 up ;" and the third sort is exuded from the leaves of the linden, 

 oak, plum, and other trees, generally in the early mornings, by 

 a change of temperature after very close, hot weather. Most 

 of the American writers on bee-keeping appear to have had 

 little or no personal experience of honey-dew. Professor Cook 

 mentions it as occurring in California, in a case where it was 

 unmistakably exuded from the tree leaves ; and Langstroth 

 quotes the English entomologists, Spence and Kirby, as to clear 

 cases of plant-louse origin ; also Bevan, who, however, does not 

 suggest anything as to the cause of the phenomenon, but merely 



" Honey-dew usually appears upon the leaves as a viscid transparent 

 substance, as sweet as honey itself, sometimes in the form of globules, 



