302 AUSTRALASIAN 
SOMETIMES THROWN OFF AS SUPERFLUOUS. 
There are, however, occasions when considerable quantities 
of such matter are thrown off, 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 (in a passage already quoted at page 86) that, 
«* 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.” 
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 off the 
leaves like drops of rain. Many kilogrammes might have been collected 
from a moderate-sized linden tree.” 
And further on, at page 141, he says :— 
‘In a hot summer, when the deficiency of moisture prevents the 
absorption 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 place 
in the leaves; and all the constituents of the leaves, including the 
alkalies and alkaline earths, must participate in effecting 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 applied in the formation 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 amongst scien- 
tific 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 
