Influence of Light on the Growth and Nutrition of Animals. 145 



3. Those processes of vegetable life and of inorganic change 



upon which the proper condition of the respiratory medium 



depends. 



Speaking generally then, it must be admitted that light is essential 

 to the development, growth, and nutrition of animals, but only in- 

 directly. In the foregoing experiments, the usual coincidents of 

 light, a proper supply of food, a due aeration of the respiratory 

 medium, a properly regulated external temperature, &c. having 

 been provided in each cell, the direct inflvience of hght only being 

 changed, no corresponding change occurred in the animal life. 



In the vegetable kingdom the case is quite different, and the ex- 

 periments on Lathyrus odorattts recorded in this paper demonstrate 

 again, what has been shown by numerous other experimenters, that 

 light as a direct agent is essential to the nutritive processes of 

 plants. An interesting exception occurs, however, in the vegetable 

 kingdom, which serves to strengthen the probability that the con- 

 clusions arrived at concerning animals are correct, viz. that fungi — 

 which derive their nutriment, like animals, from organic compounds 

 already prepared for them — perform their vital functions without 

 dependence on the influence of light. 



Under the head of colour, it would seem that the familiar phe- 

 nomenon of etiolation witnessed in plants which have been deprived 

 of light, has led to erroneous anticipations as to the effect which 

 alterations of light would produce upon the development of the 

 colouring materials in animals. 



In the experiments here recorded, it is seen that neither white 

 light, nor the different coloured rays, nor darkness altered the de- 

 velopment of those materials necessary to the exhibition of colour, 

 when the animal was seen in ordinary light. The experiments of 

 Dr. Gladstone, on plants, also show that the development of colouring 

 matter in the petals of flowers is independent of the influence of 

 light ; that flowers raised under the difl'erent coloured rays and in 

 darkness have the same colour in their petals as when raised in or- 

 dinary light. Thus, even in vegetables, etiolation is confined to 

 those parts of the plant which depend for their colour upon the 

 condition of the chlorophyl, to the green appearance of which some 

 portion of the solar beam is evidently essential. 



Although, therefore, at first sight, the results of my experiments 

 under the head of colour may appear questionable, I think we must 

 rather throw the question upon tlie correctness of our preconceived 

 notions on the subject; and the facts elicited by Prof. Edward 

 Forbes (referred to in the Paper), while retaining all their value and 

 interest as assistants in determining the depths of primeval seas, 

 cannot, I think, be taken as evidence against the correctness of my 

 observations. On the otlier hand, the results of my experiments 

 may be found to put a new construction upon the facts observed by 

 Prof. Forbes. lie discovered that increased depth of sea corre- 

 sponds with diminished light, and that both of these conditions 

 again correspond with ])ecnliar changes in colour, and ultimately 

 with loss of colour in the shells inhabiting these depths ; but there 



Phil. Mu<j. S. 4. Vol. 18. No. 118. Auij. 1859, L 



