ON THE ASSIMILATING AND ELABORATING POWERS OF PLANTS. 33 
situation, may be variously affected by electric currents, or other of the 
great, though invisible natural agents. That exposure to air, light, 
and heat has a decided maturing effect upon the juices of a plant, is 
manifest from our finding the essential qualities, in most cases, more 
concentrated in the bark and leaves than in the interior recesses of the 
system; and every one acquainted with the qualities of herbs, and vve 
may add, trees also, knows well that a dry and very exposed situation 
serves to exalt and enrich the essential qualities of any plant much 
more than in circumstances where air, and light, and solar heat are 
denied. Upland hay, cut at the proper season, is always richer in 
saccharine matter than that produced in the moist valley; and that 
from the elevated knolls of any one field is always much more salu¬ 
brious and nutritious than the produce of the lower parts. The sap 
drawn from the highest parts of a tree is richer in quality than the 
same drawn from the lower part of the trunk. This fact is always 
experienced by the manufacturers of sugar from the maple, and wine 
from the birch trees. So it is noticed that fruit, as well as herbs, are 
found sweeter in a dry than in a wet summer ; and the gardener 
always withholds water from his plants which are about to ripen their 
crops. 
All these instances show that the sap is more or less elaborated, 
according as it is more or less exposed to atmospheric influences, to the 
character of those influences, and to the extent of the vascular apparatus 
through which it has to percolate. 
It is well deserving of notice to mark the difference which is perceiv- 
able between the state of pure water ascending through dead and living 
vegetable membrane or tissue. In the former it is discharged as pure 
as it enters: in the latter it comes forth impregnated with the quali¬ 
ties of the branch or twig through which it has passed. This differ¬ 
ence can only be attributed to the vital action of the one, and the want 
of it in the other. 
A good example of the elaborating functions of plants may be shown 
from what chemists have discovered by the analysation of the common 
liquorice (Glycyrrhiza glabra), which, besides sugar, contains an 
amylaceous fecula, a crystalline substance, a resinous oil, phosphate 
of lime, and malate of lime ; and it is highly probable that there are 
some plants which elaborate even a greater number of distinct essences 
than the liquorice. 7 
That the organisation of every plant gives the essential character to 
the juices which it contains is manifest, from the circumstance that the 
sap absorbed by the roots, and conveyed by the stem of the stock, has 
VOL. V. —NO. LVs 
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