342 



THE FOOD OF PLANTS 



the assimilatory powers of the chloroplastids. In opposition to Pringsheim's * 

 supposition, all the experimental evidence tends to show that light exercises but 

 little or no influence upon respiration so long as no injurious effect is produced 

 (Sects. 104 and 56). The fact that intense light may cause a bleaching of the 

 chloroplastids in the presence of oxygen, even if it is accompanied by an increased 

 production of carbonic acid gas, hardly affords safe or direct evidence as to the 

 behaviour under normal conditions. From the erroneous hypothesis that light 

 accelerates respiration *, Pringsheim developed the theory that chlorophyll acts as 

 a protection against light, and thus allows the decomposition of carbon dioxide to 

 become more prominent in green cells than in colourless ones, for he supposed 

 that light might induce photosynthesis in colourless cells as well if only the rays 

 which accelerate respiration were extinguished. This theory hardly accords, however, 

 with the large amount of chlorophyll present in shade-loving plants, or with the 

 fact that in very feeble light respiration overpowers assimilation ; moreover we 

 should expect to find the most active assimilation occurring in light which had 

 passed through a solution of chlorophyll or through a green leaf, whereas in such 

 light colourless protoplasts and leucoplastids respir with undiminished intensity 

 and show no traces of an evolution of oxygen. Similarly, if such green light 

 is sufficiently concentrated, it may be shown by means of the bacterium method 

 that oxygen is evolved only from the chlorophyll bands of Spirogyra or Afesocarpus, 

 and not from the colourless plasma s . These facts so completely deprive Pringsheim's 

 theory of all scientific foundation that further discussion of it is unnecessary. 



SECTION 60. The Influence of the Different Hays of the Spectrum. 



Only the visible light-rays take part in the assimilation of carbon 

 dioxide, except in purple bacteria, in which assimilation is most active in the 

 neighbouring but invisible infra-red rays. A correct knowledge of the 

 assimilatory effect of the different regions of the spectrum can only be 

 obtained by determining the amounts of carbon dioxide decomposed by the 

 superficial chloroplastids, for the more deeply seated ones receive light of 

 altogether different composition to that which falls upon the outer surface. 

 The former gives the primary curve of assimilation which Engelmann 4 

 obtained by means of the bacterium method, whereas the estimation of 

 the total assimilation yields the secondary curve. The primary curve 



1 Pringsheim, Jahrb. f. wiss. Bot., 1879-81, Bd. xn, p. 374; Ber. d. Bot. Ges., 1886, Bd. IV, 

 p. Ixxxiv. 



" [Kolkwitz has, however, found (Jahrb. f. wiss. Bot, 1898, Bd. XXXIII, p. 128) that light 

 distinctly accelerates respiration in fungi, though the increase is not more than a tenth of the 

 previous amount.] 



* Engelmann, Bot. Zeitung, 1883, p. 5 ; Die Erscheinungsweise der Sauerstoffausscheidung im 

 Licht, 1894, Fig. 7 (Sep.-abdr. a. d. Verb. d. Amsterd. Akad.). See also Reinke, Bot. Zeitung, 



883, P. 733- 



4 Engelmann, Bot. Zeitnng, 1882, p. 419; 1883, p. i ; 1884, p. 80; 1886, p. 64; 1887^.393; 

 Die Erscheinungsweise, &c., 1894 (Sep.-abdr. a. d. Verb. d. Amsterd. Akad.). 



