346 THE FOOD OF PLANTS 



a thin semi-transparent green leaf absorbs, as a general rule, more than half of the 

 total energy of the sunlight falling upon it, and the rays most active in assimilation 

 are almost entirely extinguished, so that the sunlight which has passed through a green 

 leaf is usually no longer able to induce a formation of starch or an evolution 

 of gas bubbles '. For the absorption in the Florideae and Cyanophyceae, see Figs. 

 51 and 52, and the literature quoted. 



Even in passing through colourless tissues a large amount of light is lost by 

 reflection and absorption, and when coloured pigments are present the amount 

 absorbed is much increased. Engelmann has shown that the red dyes frequently 

 present in the cell- sap of chlorophyllous leaves absorb fewest of the rays which 

 are most active in assimilation (red and orange), and most of the less active 

 yellow rays. The total amount of assimilation is, however, decreased, and hence 

 apparently arises the fact that copper-beeches grow more slowly than the green 

 varieties 2 . 



The value of either monochromatic or mixed light which reaches a 

 chlorophyllous cell is dependent upon its concentration and the specific 

 assimilatory energy of the component rays, and the assimilatory effect rises 

 with an increased concentration so long as the illumination is of sub- 

 maximal intensity (Sect. 59). The specific assimilatory energy remains 

 the same for any given light-rays, whether they act alone or in combina- 

 tion with other rays, as is shown by the results obtained with mono- 

 chromatic light, as well as by special researches 3 ; but although assimilation 

 may continue in monochromatic light, still in the absence of all other 

 rays the growth and general vital activity of a plant may ultimately be 

 injuriously affected. 



The fact that the primary assimilatory curve is different in green 

 plants to what it is in red seaweeds or purple bacteria shows that the 

 assimilatory mechanism may exhibit specific peculiarities, and it is not 

 impossible that organisms may be discovered in which the greatest as- 

 similatory activity may be produced by the short ultra-violet rays, or 

 by the rays with longest wave-length. Various rays of the spectrum 

 can exert photo-chemical actions on dead substances, and although it is 

 those of shorter wave-length which are especially active, still it is a funda- 

 mental error to construct a curve representing the general chemical intensity 

 of the different rays of light based upon their action on silver salts 4 . 

 Similarly, since the different rays have a definite specific assimilatory 



Bot. Zeitung, 1884, p. 87; 1887, p. 441 ; 1888, p. 685; Reinke, ibid., 1886, p. 161 ; Monteverde, 

 Acta horti Petropolitani, 1893, Bd. xill, p. 123; Tschirch, Ber. d. Bot. Ges., 1896, p. 76. Other 

 literature is given in these works. 



1 Nagamatsz, Arb. d. Bot Inst. in Wiirzburg, 1887, Bd. Ill, p. 399. 



a Jumelle, Compt. rend., 1890, T. cxi, p. 380; Engelmann, Bot. Zeitung, 1887, p. 433. 

 Absorption, &c. by colourless tissues : Reinke, Bot. Zeitung, 1886, p. 193 ; Engelmann, 1. c., p. 413. 



3 Pfeffer, Arb. d. Bot. Inst. in Wurzburg, 1871, Bd. I, p. 45. 



4 Ostwald, Lehrb. d. allgem. Chemie, 1893, Bd. II, i, p. 1024. 



