• Volume 58, Issue 2

      August 1963,   pages  65-116

    • Floral colours and the physiology of vision - Part I. Introductory

      C V Raman

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      Monochromatic light is composed of energy-quanta which are all equal. Our perception of colour is thereby brought into the closest relationship with the notions of the quantum theory. Polychromatic light, however, stands on a different footing and the problem of the colour which it exhibits demands separate consideration. The relationship between the spectral composition of such light and its observed hues can only be ascertained only by the observational study of a great number of cases. It is pointed out that a vast mass of material exhibiting colour and highly suitable for investigations of this nature presents itself to us in the products of the biological activity of trees and plants,viz., their leaves, flowers and fruits.

    • Floral colours and the physiology of vision - Part II. The green colour of leaves

      C V Raman

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      The light emerging with sensible intensity after passage through the material of a green leaf is observable in the spectral regions of green and red extending from 520 mµ to 645 mµ. These limits are set by the carotenoid and the chlorophyll pigments in the leaf which exercises powerful absorptions at smaller and at greater wavelengths respectively. The characteristic absorption bands of the chlorophylls can be seen under strong illumination at the extreme red end of the spectrum. But they do not sensibly influence the observed colour of the leaf, since the extreme red is of very low luminous efficiency. Actually, the operative cause which determines the observed colour of the leaf is the absorption by the leaf pigments manifested in the yellow region of the spectrum between 570 mµ and 590 mµ. The colour of the leaf appears a deeper green with increasing strength of that absorption.

    • Floral colours and the physiology of vision - Part III. The spectrum of the morning glory

      C V Raman

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      The flowers of the Morning Glory (Ipomea learii) exhibit a bright blue colour of almost spectral purity in daylight. Visual observations, confirmed by spectrum photographs, show that this visual sensation results from the removal of the yellow and orange radiations from the complete spectrum, the relative intensities of the red, green and blue-violet regions remaining unaltered. The effect observed may be interpreted as the result of the masking of the visual effect of the red and green sectors by the blue sector in the spectrum in these circumstances.

    • Floral colours and the physiology of vision - Part IV. The queen of flowers

      C V Raman

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      The flowering treeLagerstroemia Flos Reginae (also known asL. Speciosa) has two varieties bearing purple and pink flowers respectively. Spectroscopic examination shows that the purple colouration represents the entire spectrum of white light in which the limited region from 570 mµ to 590 mµ is absent. The pink colour results from a weakening of the spectrum in the range of wavelengths from 520 mµ to 570 mµ.

    • Floral colours and the physiology of vision - Part V. The blue of the jacaranda

      C V Raman

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      Spectroscopic observations show that the blue colour of the flowers of the Jacaranda tree is ascribable entirely to two weak absorption bands, one appearing in the yellow and the other midway between the orange and red sectors, the rest of the spectrum of white light showing no other observable variation from its normal distribution of intensity.

    • Floral colours and the physiology of vision - Part VI. Comparative study of three cases

      C V Raman

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      The spectral composition of the colours exhibited by the flowers ofSolanum grandiflorum, Nymphaea caerulea andThunbergia grandiflora has been studied. The results are surprising and an explanation is offered for them in terms of the masking or blocking out of the visual effect of the longer wavelengths in the spectrum by the shorter wavelengths, the necessary condition for which is the removal of the yellow sector, accompanied by a weakening in intensity of the red end of the spectrum.

    • Floral colours and the physiology of vision - Part VII. The aster and its varied colours

      C V Raman

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      Asters exhibiting vivid colours may be placed in two strikingly contrasted groups, each containing three sub-groups. Spectroscopic examination shows that the perceived colours of one group ranging from purple to violet arise from an extinction of the yellow, while the colours ranging from pink to rose-red of the other group are the result of an absorption in the green.

    • Floral colours and the physiology of vision - Part VIII. The spectra of the roses

      C V Raman

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      The characteristic hue designated as rose-red arises from a suppression of the green sector in the spectrum, the radiations of both longer and shorter wavelengths coming through freely. The more complete the absorption of the green is, the more saturated does the resulting hue appear. The reason for the dominance of the red sensation and the saturated hues observed in these circumstances with many varieties of roses is discussed.

    • Floral colours and the physiology of vision - Part IX.Hibiscus andBougainvillea

      C V Raman

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      The spectroscopic behaviours of the flowers ofHibiscus syriacus andHibiscus rosa sinensis are quite different. The purple colour in the former case is due to an absorption in the yellow and the red in the latter to an absorption in the green. The purple and red varieties ofBougainvillea show differences of the same general nature and having a similar origin.

    • Floral colours and the physiology of vision - Part X. Flowers exhibiting band spectra

      C V Raman

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      The absorption spectra of the blue flowers ofClitoria ternatea and of the purplish-red flowers of the orchidAerides multiflorum, have been recorded. They show a striking similarity with the absorption spectra of the petals ofCineraria exhibiting those colours. Various yellow flowers exhibit a banded structure in the blue-violet regions of the spectrum which they only feebly transmit.

    • Floral colours and the physiology of vision - Part XI. A review of the results

      C V Raman

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      The results emerging from the study of the individual cases have been brought together in this part of the memoir and illustrated by further examples. In particular, the special role in the physiology of vision played by the yellow region of the spectrum and the remarkable manner in which its presence or absence determines the character of the sensory impressions produced by polychromatic radiation are described. It also emerges that in certain circumstances, the sensory effect of the two parts of the spectrum on either side of the yellow may be masked or suppressed, one by the other, orvice versa.

    • Floral colours and the physiology of vision - Part XII. Some concluding remarks

      C V Raman

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      The results of the present studies on the visual perceptions of polychromatic radiation support the fundamental thesis that the primary physiological sensations are those excited by monochromatic light. The so-called trichromatic hypothesis and the theories of colour vision based on it are not logically sustainable. They are further contradicted by the facts of observation described in the present memoir.

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