75 



SOMETIMES THROWN OFF AS SUPERFLUOUS. 

 There are, however, occasions when considerable quantities of such 

 matter are thrown ofE or exuded by the leaves, which effect is taken to 

 indicate an abnormal or unhealthy condition of the plant. At pages 106 

 and 107 of Liebig's book (speaking of an experiment made to induce the 

 rising sap of a maple-tree to dissolve raw sugar applied through a hole cut 

 in the bark) he shows that, — 



When a sufficient quantity of nitrogen is not present to aid in the assimi- 

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

 excrements from the bark, roots, leaves, and branches. 



In a note to this last paragraph we are told that — 



Langlois has lately observed, during the dry summer of 1842, that the 

 leaves of the linden-tree became covered with a thick and sweet liquid in 

 such quantities that for several hours of the day it ran ofi the leaves like 

 drops of rain. Many kilograms might have been collected from a moderate- 

 sized hnden-tree. 



And further on, at page 141, he says, — 



In a hot summer, when the deficiency of moisture prevents the absorp- 

 tion of alkalies, we observe the leaves of the lime-tree, and of other trees, 

 covered with a thick liquid containing a large quantity of sugar ; the carbon 

 of the sugar must, without doubt, be obtained from the carbonic acid of the 

 air. The generation of the sugar takes places in the leaves, and all the con- 

 stituents of the leaves, including the alkalies and alkahne earths, must 

 participate in efiecting its formation. Sugar does not exude from the leaves 

 in moist seasons, and this leads us to conjecture that the carbon which 

 appeared as sugar in the former case would have been apphed in the forma- 

 tion of other constituents of the tree in the event of its having had a free 

 and unimpeded circulation. 



These quotations will probably be considered sufficient to justify the 

 assertion that the gathering of the honey from plants can in no possible 

 way tend to exhaust the soil or affect its fertility. There is no difference 

 of opinion among scientific men as to the sources from which the saccharine 

 matter of plants is derived. Since Liebig first put forward his views on that 

 subject, as well as with regard to the sources from which the plants derive 

 their nitrogen, the principles of agricultural chemistry have been studied 

 by the most eminent chemists, some of whom combated the views of Liebig 

 on this latter point (the source of nitrogen and its compounds), and Liebig 

 himself seems to have modified his views on that point ; but there has-been 

 uo difierence of opinion about the saccharine matter, as to which Liebig's 

 doctrine will be found given unaltered in the latest colonial work on the 

 subject, Maclvor's " Chemistry of Agriculture," published at Melbourne a 

 few years ago. 



