IN NATURE, vol. xiii. p. 348, Mr. Thiselton Dyer gives an extract from Sachs's “Text-book of Botany” to the effect that no change is produced in the colour of flowers by growing them in the dark. This led to a letter from Mr. J. C. Costerus (vol. xiii. p. 427), calling attention to the results obtained by Askenasy, published in 1876, who found that some kinds were changed in colour, and some not changed. In the autumn of 1873 I made a number of experiments on this subject, and published a short account of them in the same year in the Quarterly Journal of Science, vol. iii. p. 474. I came to the same conclusion as Askenasy has come to, and was also able to establish some important generalisations. I will only mention a few special instances. I was unable to cause any change in the colour of the common Orange Lily (Lilium aurantiacum), whereas I found that a very considerable change was produced in the case of Erysimum Peroffskianum by only a moderate degree of darkness. This may perhaps be owing to the fact that the orange tint of these two kinds of flowers is due to entirely different substances. That of the lily is due to what I have called orange xanthophyll, whereas that of the other flower is due to a much less stable compound, giving an entirely different spectrum, met with also in the orange marigold, and therefore named by me Calendula xanthine. Comparing together the mixed colouring matters found in an equal weight of the petals, I found that the amount of the Calendula xanthine was only half as great in the petals grown in the dark, whereas the more yellow constituents were reduced only to three-quarters, so that the general colour was more yellow. I found that a similar change could be produced in the case of the marigold. If shaded when the flowers are somewhat grown, the total colour may be very considerably reduced without there being any material change in the ratios of the different colouring matters, whereas when grown in the dark from a very small bud, the ratios are changed, as in the case of Erysimum. Growing flowers in the dark seems to stop the normal development to a greater or less extent according to the nature of the colouring-matters, the effect being the greatest in the case of those substances which are the most easily decomposed. We thus find what appears at first sight to be a very unlikely result, viz., that those constituents which, when dissolved out from the petals, are the most easily discolorised by exposure to light, are formed in relatively greater amount when the flowers are grown in the light, which is easily explained if we assume that a higher vital power, depending on the presence of light, is necessary to overcome the more powerful chemical affinities of the less stable compounds.
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