THE ACTION OF LIGHT 567 



light energy were converted into heat in the chlorophyll itself, a temperature of 

 6,000 C. would be obtained. Of course, the energy is converted into chemical work, 

 without passing through heat, but the calculation gives us an idea of the intensity 

 of the energy changes brought into play. It may be noted that carbon dioxide is 

 dissociated into carbon monoxide and oxygen at about 1,200, but this fact 

 would not assist the comprehension of the photo-chemical process, even if we 

 admit that such a temperature might be attained locally in the chloroplast, since 

 the union of carbon monoxide and hydrogen to produce formaldehyde requires the 

 action of light. 



The reduction of carbon dioxide can, nevertheless, be effected by certain photo- 

 plasmic systems by the aid of chemical energy without light, so that the photo- 

 synthetic process is not to be regarded as an altogether singular one. Other forms 

 of energy, besides that of light, can be utilised by certain organisms for the purpose. 

 For example, there are some bacteria which can use the energy obtained by the 

 oxidation of hydrogen to reduce carbon dioxide for their own carbon needs. 

 Suppose we grow these bacteria in a closed vessel, containing hydrogen and oxygen, 

 we find that combustion of the gases to form water takes place. If carbon dioxide 

 is also present, it simultaneously disappears and the carbon is assimilated by the 

 bacteria. If hydrogen and carbon dioxide alone are present, there is a slight 

 disappearance of both, but very little. 



Electrical Changes. Such have been described in the green leaf in consequence 

 of illumination. The work of Haacke (1892) and of Waller (1900, 2) may be 

 mentioned. The effects appear to be connected with the photo-synthetic process, 

 since they are absent if the light has already been deprived of the rays absorbed 

 by chlorophyll by passing through another green leaf previously. These effects 

 are nearly as great when red light is used as when white light is used. Waller 

 has shown that removal of carbon dioxide abolishes the response, and that it can be 

 brought back by adding carbon dioxide to the atmosphere in which the leaf is 

 situated. The fact of an electrical response is of interest in connection with the 

 electronic theory of photo-chemical change, described above (page 552), but cannot as 

 yet be explained. Harvey Gibson and Titherley (1908) have suggested an electro- 

 chemical theory of chlorophyll assimilation on the basis of these electrical effects. 



Red and Brown Seaweeds. The absorption of light by chlorophyll, as we have 

 seen, is such as to make the best use of the light available. But a green pigment 

 is, of course, transparent to the green rays, which preponderate under water, so 

 that it would be inefficient in that situation. Accordingly, as Engelmann has 

 pointed out (1882, 2), we find, in the seaweeds, red and brown pigments corre- 

 sponding to chlorophyll and having the same function, but able to absorb effectively 

 the green light available. For example, the red seaweeds show a maximum of 

 carbon assimilation in the green and, spectrophotometrically measured, they show 

 the greatest absorption in the same region, although there is also considerable 

 absorption between the lines B and C, where the chief band of chlorophyll lies. 

 The minimum of absorption is in the orange between C and D (p. 220 of the paper 

 referred to). This fact serves to illustrate the function of chlorophyll as an optical 

 sensitiser ; the same effect is produced by light of various wave lengths, provided 

 that it is absorbed. 



In certain cases, to which Engelmann has given the name of " complementary 

 chromatic adaptation" we find that a pigment is actually formed under the action 

 of coloured light and that the pigment has a colour which is complementary to 

 that of the light to which the organism is exposed, so that this light is then 

 absorbed. The alga, Oscillaria sancta, as shown by the work of Gaidukov (1902), 

 occurs in several colours between reddish purple and blue-green. If cultures are 

 made, say, of the reddish-purple variety, we find that under red light a green 

 pigment is produced. If we take the green variety, it becomes reddish under 

 green light, brownish yellow under blue light, and so on. The general colour of 

 a mixed culture thus tends to become complementary to that of the light under 

 which it is grown. The work was done with great care and spectro-photometer 

 curves of the various pigments were compared with those of the light under which 

 they made their appearance. 



