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 

 refiangibility. 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 0/ light on plants in proportion to its intensity^. That 



^ Sachs, Bot. Zeitg, 1864, p. le^z *' seq., where the labours of previous observers are referred to 

 in detail. 



^ I have replied, in the second part of the * Arbeiten dee botan. Inst, in Wurzburg,' 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 



