Professor A. Vogel, in a communication to the "Sitzungsberichte der
Munchener Akademie," brings into prominence the fact that the hemlock
plant, which yields coniine in Bavaria, contains none in Scotland.
Hence he concludes that solar light plays a part in the generation of
the alkaloids in plants. This view is corroborated by the circumstance
that the tropical cinchonas, if cultivated in our feebly lighted
hothouses, yield scarcely any alkaloids. Prof. Vogel has proved this
experimentally. He has examined the barks of cinchona plants obtained
from different conservatories, but has not found in any of them the
characteristic reaction of quinine. Of course it is still possible
that quinine might be discovered in other conservatory-grown
cinchonas, especially as the specimens operated upon were not fully
developed. But as the reaction employed indicates very small
quantities of quinine, it may be safely assumed that the barks
examined contained not a trace of this alkaloid, and it can scarcely
be doubted that the deficiency of sunlight in our hothouses is one of
the causes of the deficiency of quinine.
It will at once strike the reader as desirable that specimens of
cinchonas should be cultivated in hothouses under the influence of the
electric light, in addition to that of the sun.
If sunlight can be regarded as a factor in the formation of alkaloids
in the living plant, it has, on the other hand, a decidedly injurious
action upon the quinine in the bark stripped from the tree. On drying
such bark in full sunlight the quinine is decomposed, and there are
formed dark-colored, amorphous, resin-like masses. In the manufacture
of quinine the bark is consequently dried in darkness.
This peculiar behavior of quinine on exposure to sunlight finds its
parallel in the behavior of chlorophyl with the direct rays of the
sun. It is well known that the origin of chlorophyl in the plant is
entirely connected with light, so that etiolated leaves growing in the
dark form no chlorophyl. But as soon as chlorophyl is removed from the
sphere of vegetable life, a brief exposure to the direct rays of the
sun destroys its green color completely.
Prof. A. Vogel conjectures that the formation of tannin in the living
plant is to some extent influenced by light. This supposition is
supported by the fact that the proportion of tannin in beech or larch
bark increases from below upward--that is, from the less illuminated
to the more illuminated parts, and this in the proportions of 4:6 and
5:10.
Sunny mountain slopes of a medium height yield, according to wide
experience, on an average the pine-barks richest in tannin. In woods
in level districts the proportion of tannin is greatest in localities
exposed to the light, while darkness seems to have an unfavorable
effect. Here, also, we must refer to the observation that leaves
exceptionally exposed to the light are relatively rich in tannin.
We may here add that in the very frequent cases where a leaf is
shadowed by another in very close proximity, or where a portion of a
leaf has been folded over by some insect, the portion thus shaded
retains a pale green color, while adjacent leaves, or other portions
of the same leaf, assume their yellow, red, or brown autumnal tints.
If, as seems highly probable, these tints are due to transformation
products of tannin, we may not unnaturally conclude that they will be
absent where tannin has not been generated.--_Jour. of Science._
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