CHLOROPHYLL: LIGHT ABSORPTION 129 



chloroplasts of all flowering plants, and in fairly constant 

 proportions. We need not here go into the theories of the 

 way in which the chlorophyll complex acts, but we must con- 

 sider its relation to Hght absorption, and some related points. 

 The green colour indicates that when white light passes 

 through chlorophyll both the blue and the red rays are 

 absorbed, allowing the green to pass on. Spectroscopic 

 examination shows that chlorophyll possesses four absorption 

 bands in the orange end of the spectrum, the strongest lying 

 between the B and C lines, and three at the blue end. A 

 familiar demonstration experiment of timing the bubbles 

 emitted by Elodea, exposed to sunlight behind red and blue 

 screens, shows that under these conditions the red Hght is 

 much more active in assimilation than the blue. The experi- 

 ment is very faulty, because no account is taken of the relative 

 strength and purity of the light of the two colours, but, in 

 fact, it is certain that in sunlight the red end of the spectrum 

 is most effective. To determine accurately the effective- 

 ness of the different portions of the spectrum, and to relate 

 them to the absorption by chlorophyll at the corresponding 

 points, and to the energy of the light, is so difficult a matter 

 that, despite repeated attempts, no satisfactory result has 

 yet been attained. Pfeffer found maximum assimilation in 

 the red at the C absorption band; Engelmann (1883) found 

 maximum assimilation near the same point, with a second 

 lesser maximum in the blue ; Kniep and Minder (1909) 

 found red and blue light of equal energy to produce the 

 same rate of assimilation, while green produced none. Most 

 recently Ursprung (19 17), in an investigation carried out with 

 all physical precautions, found that assimilation occurred in 

 every region of the spectrum, in proportion to the energy 

 of the light and to the extent of its absorption by the chloro- 

 phyll ; unfortunately, his estimate of the amount of assimi- 

 lation was based only on a rough colorimetric estimate of 

 the starch formed. From such results we can draw no 

 definite conclusions ; we can only say that the balance of 

 the evidence is in favour of an important utilisation of blue 

 as well as of red light. 



K 



