126 METABOLISM 



owing to the deficiency in carbon-dioxide. Carbon-dioxide may be present in 

 sufficient quantity under ordinary circumstances to employ all the energy of 

 sunlight, but when light is artificially increased the usual amount of carbon- 

 dioxide would constitute a sub-optimum. Further investigations may furnish 

 us with a graphic curve representing the relation existing between assimila- 

 tion and hght intensity, similar to those which express graphically so many 

 physiological processes, and showing a descending curve with further increase 

 in light intensity, after a maximum point has been reached. So far such 

 descending curves as those obtained by Reinke have been established only 

 for lights of high intensity, which might inhibit the assimilative activity by 

 injuring the chloroplasts. 



[Pantanelli (1904) has shown that in Elodea the optimum decomposition 

 of carbon-dioxide takes place (when the water contains a normal percentage 

 of that gas) when the light intensity amounts to one-quarter of that of direct 

 sunlight, but as the light intensity is increased assimilation decreases. Doubt- 

 less plants with varying requirements so far as light is concerned (' shade- 

 plants ' and ' light-plants ') behave differently in this respect (compare Weiss, 

 Compt. rend. 1903). Attention has already been drawn (p. 120) to the fact 

 that Pantanelli has shown that the optimum intensity of light is higher as 

 the amount of carbon-dioxide in the environment increases.] 



Sunlight is known to be a mixture of rays of different refrangibility, of 

 different wave length and of different colour, and it was long ago a question 

 whether all these rays act in the same way or not. On this question much 

 has been written, but the results which have been arrived at are by no means 

 in proportion to the labour expended on them, on account of the difficulties 

 that had to be overcome — or more correctly, perhaps, that have yet to be 

 overcome, for no final answer to the problem has even yet been arrived at. 

 The first researches on the subject were those of Daubeny (1836) ; this investi- 

 gator grew plants behind coloured glass plates, and this method is still of great 

 service for demonstration purposes, although to be rejected for exact research, 

 since the light so obtained is by no means monochromatic. We are unable to 

 make much further progress by using coloured solutions, since although the 

 light rays so filtered are virtually monochromatic (Landolt, 1894), still they 

 are necessarily very feeble and consequently of little use physiologically. 

 Hence in all recent research of an exact character the use of coloured media 

 has been quite subsidiary to that of light subdivided into its components 

 by means of a spectroscope. Draper (1843) was the first to put the solar 

 spectrum to this use, and Pfeffer (1872), Reinke (1884), Engelmann 

 (1884), Timiriasef (1885), and others have made use of the same method. 

 It cannot be described, however, without qualification, as an exact method, 

 since the way in which the spectrum is produced involves the much wider dis- 

 persion of the more refrangible rays, so that equal areas in different regions 

 of the spectrum cannot be equally effective. On the other hand, in order to 

 increase the intensity of the light the slit of the apparatus must be opened so 

 widely that the spectrum becomes no longer a pure one. The first source of 

 error may be avoided by producing a normal spectrum with the aid of a dif- 

 fraction grating. Such experiments have not yet been carried out, though 

 Reinke (1884) has devised an apparatus by which comparable observations 

 may be made without the aid of a diffraction grating. The principle of this 

 ' spectrophore ' (Fig. 26) is as follows : — 



Light from a heliostat is made to converge by means of a lens 0, and to 

 pass through the prism P, by which it is decomposed. The spectrum falls on 

 the screen SS^. By means of two strips of wood {DD^) selected parts of the 

 spectrum may be blotted out, and the remainder of the rays are combined by 

 means of the lens 5 into a beam of light, which is made to play on the plant. 



