ACTION OF LIGHT ON VEGETATION. 66 1 



On the other hand the rays of high refrangibility (the blue or violet, as well 

 as the invisible ultra-violet rays) are the principal or the only ones which produce 

 mechanical changes so far as these are dependent on light. It is these rays which 

 influence the rapidity of growth, alter the movements of the protoplasm, compel 

 swarm-spores to adopt a definite direction in their motion, and change the tension 

 of the tissues of the motile organs of many leaves and hence affect their position. 



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 terchloride, 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 the 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 these 

 fluids is directed on plants capable of decomposing carbon dioxide, 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 colouring 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 con- 

 trary, proceeds in this light exactly as in the dark, although the leaves turn green. 

 Conversely the light which had passed through the ammoniacal copper oxide had 

 very little effect in decomposing carbon dioxide, although the action on photo- 

 graphic paper was very vigorous. The growth of seedlings was on the coritrar\- 

 the same as in white light ; and the mechanical process of heliotropic curvature was 



' Sachs, Bot. Zcit. 1864, p. 253 ei seq., where the labours of previous observers are referred to 



ill detail. 



