INFLUENCE OF DIFFERENT FACTORS ON PIGMENT FORMATION 427 



tion, that is, whether the absorption spectrum of the green pigment is 

 particularly suitable for this purpose. Timiriazev (1883), in particular, 

 saw a proof of this adaptation in the (alleged) coincidence of the absorp- 

 tion peak of chlorophyll with the intensity peak of the solar spectrum. 

 He has been rebuked by Engelmann (1884), because only by an arbitrary 

 interpretation of the solar energy distribution curve (c/. Vol. II, Chapter 

 22) can the maximum of the latter be placed near 670-680 m/x. (This 

 criticism did not prevent Timiriazev and his pupils from continuing to 

 assert that the coincidence of the absorption maximum of chlorophyll 

 with the intensity maximum of the sun spectrum is a striking argument 

 in favor of Darwinian theoiy.) 



A more elaborate attempt to explain the color of plants as the result 

 of chromatic adaptation was made bj^ Stahl (1909), who suggested that 

 the existence of the two absorption peaks of chlorophyll — one in the red 

 and one in the violet — is particularly favorable because of the two types 

 of illumination to which the plants are subjected — direct sunlight wdth 

 its maximum in the yellow, and sky light with its maximum at the violet, 

 end of the visible spectrum. Stahl attributed the fact that the first 

 absorption maximum of chlorophyll lies in the red rather than in the 

 yellow to the desire of the plant to balance the light energy absorbed in 

 the sun and in the shade. An absorption maximum too near the intensity 

 maximum of the sun would cause the absorption of an excessive amount 

 of energ}^, thus leading to overheating and injury. However, an equally 

 ingenuous explanation could probably be found for any other arrangement 

 of the absorption bands. Such speculations can be answered simply by 

 pointing out that the intensity of sunlight changes comparatively slightly 

 over the whole visible spectrum, and that average leaves absorb 50 to 

 80% of the light throughout this whole region (c/. figures in Chapter 22, 

 Vol. II). The only spectroscopic property of chlorophyll — apart from the 

 basic fact that it is a pigment — which could be considered as especially 

 favorable for its function in the plants is transparency in the near infra- 

 red. This transparency prevents the cells from absorbing light which 

 would be useless for photosjmthesis (because of the insufficient energy 

 content of its quanta). For the rest, the choice of chlorophyll as the 

 main photosynthetic pigment must be due to its photochemical properties 

 rather than to its absorption spectrum. 



4. Influence of Different Factors on Pigment Formation 



A higher plant, growing from seeds, must synthesize all its pigments 

 (except for a small quantity of carotenoids available in the seed). The 

 pigment synthesis is a complex process, and what is usually observed is 

 only the last stage, the "greening," i. e., the transformation of a colorless 

 "precursor" into the pigment. The efficiency of pigment synthesis — 



