35 



THE FOOD OF PLANTS 



Florideae fluoresce only when they are killed, and hence phycoerythrin can 

 hardly be of importance as an agency by which the wave-length of the 

 light-rays may be altered. Similarly chlorophyll appears to fluoresce but 

 little if at all in the living chloroplastid l . The solution of these and 

 similar problems may enable us to determine whether chlorophyll acts as 

 a sensitizer, enabling the light-rays to generate such molecular vibrations in 

 the active parts of the chloroplastid as to render possible the decomposition 

 of carbon dioxide 2 . 



History and Methods. Daubeny (1836), Draper (1844), Cloez and Gratiolet 

 (1851), and Sachs (1864), all found that but little assimilation occurred in blue and 

 violet light, while Draper and Pfefler found a certain correspondence existed 

 between the amount of assimilation and the brightness of the light. N. J. C. 

 Miiller (1872) found that the most active assimilation occurred in the yellow rays, 

 while the exact researches of Reinke gave a maximum between B and c, but did 



not indicate any secondary 



fir maximum in the blue 



(cf. Ass. Green, Fig. 51). 

 Since in all these experi- 

 ments only the total assi- 

 milation was observed, it 

 is not surprising that the 

 results obtained varied with- 

 in wide limits 3 . 



The nearest approach 

 to an accurate primary 

 curve was obtained by 



Engelmann, by throwing a spectrum upon an algal filament, or on a single cell, and 

 using the bacterium method as a test for the evolution of oxygen (Sect. 52). By- 

 narrowing the slit of the micro-spectroscope * the concentration of the light may be 

 lowered until an evolution of oxygen is made perceptible by the movement of the 

 bacteria and their accumulation only in the most efficient regions of the spectrum 

 (Fig. 53). In this manner it is easy to see when the illuminated side of the algal 

 filament is examined that the most active evolution of oxygen occurs opposite B-C, 

 and that the secondary maximum at F is much less obvious. In addition to this 

 method, Engelmann also used that of successive observations, by placing an algal 

 filament transversely to the spectrum, and determining in each region of the 

 latter the width of the aperture at which the movement of the bacteria first 



FIG. 53. Filament of Oedogoniunt sf>. The greatest accumulation of 

 the bacteria is bet ween B and Copposite to the dark absorption band shown 

 on the filament. 



1 Reinke, Bot. Zeitung, 1886, p. 179; Ber. d. Bot. Ges., 1883, p. 405, and the literature there given. 



2 Cf. Reinke, Bot. Zeitnng, 1886, p. 241. 



3 Pfeffer, Arb. d. Bot. Inst. in Wiiizburg, 1871, Bd. I, p. I ; Bot. Zeitung, 1872, p. 425. Here 

 and by Reinke (Bot. Zeitung, 1884, p. i) the literature is given. See also the first edition of this 

 book, p. 216; Timiriaseff, BoL Jahresb., 1875, p. 779; Ann. d. sci. nat, 1885, vii. se"r.,T. II, p. 99 ; 

 and the criticism by Reinke, Ber. d. Bot. Ges., 1885, p. 337. 



2 See Engelmann, Bot. Zeitung, 1882, p. 419, and Zeiss's Catalogue (Jena). 



