ACTION OF LIGHT ON VEGETATION. 
739 
These two laws, the result of careful observation, are only in apparent 
contradiction to the division of the rays of light which is current in chemistry 
and physics into those called chemically active, including the highly refrangible 
blue, violet, and ultra-violet, and the chemically inactive, or at least less active, 
including the less refrangible red, orange, and yellow, and partly also the green 
rays. This division has long been familiar ; silver- salts, nitrogen chloride, and 
other inorganic compounds, are powerfully acted on by the former, scarcely at 
all by the latter. But when it was shown that the organico-chemical processes 
in plants were caused mainly or solely by the latter kind of rays, it was seen that 
this classification into chemical and non-chemical rays resulted from an imperfect 
induction, and that the correct statement of the fact is rather that there are chemical 
processes (generally dependent on light) which are related to rays of particular 
refrangibility. As far as concerns the mechanical effect on the plant of the highly 
refrangible rays, it is at present uncertain whether they are not ultimately due to 
chemical changes. In any case the action is visible to the observer only in the 
form of mechanical effect (movements, tensions, &c.); and this is in harmony 
with the classification given above. 
If sunlight is made to pass through sufficiently thick strata of solutions of 
potassium bi-chromate and ammoniacal copper oxide ^, the first only permits the 
passage of light consisting of the less refrangible half of the spectrum (red, orange, 
yellow, and some green), while the blue solution allows, in addition to some green, 
only the blue, violet, and ultra-violet rays to pass through. The sunlight is therefore 
in each case halved by absorption in such a way that the spectrum beneath the 
orange solution extends from the red to the green, that beneath the blue solution 
from the green to the ultra-violet. If the light after passing through one or other of 
these fluids is directed on plants capable of decomposing carbon dioxide and of curving 
heliotropically, and pieces of very sensitive photographic paper are at the same time 
exposed by their side, it is seen that the less refrangible rays of light (transmitted 
through the potassium bichromate) effect the decomposition of carbon dioxide and 
the colouration and decolouration of the chlorophyll almost as energetically as white 
daylight, while they produce only a very slight effect on the photographic paper. 
The growth of seedlings, on the other hand, proceeds in this light exactly as in the 
dark, although the leaves turn green. Conversely the light which has passed through 
the ammoniacal copper oxide has very little effect in decomposing carbon dioxide, 
although the action on photographic paper is very vigorous. The growth of seed- 
lings is on the other hand the same as in white light ; and the mechanical process of 
heliotropic curvature is very manifest. A number of more recent observations have 
confirmed and extended the results previously obtained^. 
(2) Variatio?i in the action of light on plants in proportion to its intensity"^. That 
* Sachs, Bot. Zeitg. 1864, p. 253 et seq., where the labours of previous observers are referred to 
in detail. 
^ I have replied, in the second part of the 'Arbeiten des botan. Inst, in Würzburg,' 1872, 
to the objections urged by Prillieux to this statement, which rest on an entire confusion of the 
ideas Intensity of Light (objective), Brightness (subjective), Refrangibility (an objective), and Colour 
(a subjective) property of light. 
^ With respect to the distinction which must here be borne in mind between the objective 
3 B 2 
