THEORETICAL 



353 



Influence of coloured light. Plants may be unable to develop normally in 

 monochromatic or even mixed coloured light, although energetic photosynthesis 

 is possible so long as the chloroplastids remain healthy. Thus exposed to the 

 less refrangible end of the spectrum (behind a screen of potassium bichromate 

 solution) plants grow and increase in dry weight, but behave in other respects as 

 if in deep shade owing to the absence of the blue and ultra-violet rays which 

 prevent excessive growth and etiolation. Similarly in some cases the absence of 

 the ultra-violet rays seems to retard or inhibit the development of flowers. The 

 blue and ultra-violet rays are of little use for assimilation, and hence green plants 

 grown in light passed through cuprammonia die sooner or later for want of food. 



Hence the absence of certain rays may. ultimately injure or even cause the 

 death of a plant, and it is possible that monochromatic light may indirectly act in 

 a similar manner, although no direct injurious action is exercised by the rays 

 concerned either alone or when in mixed light. As a matter of fact certain 

 results seem to indicate that monochromatic green light does act upon plants in 

 some such manner, but further proof is needed *. The researches upon the growth 

 of plants in coloured light which have been carried out by Hunt, Sachs, A. Mayer, 

 R.Weber, Morgen, and Wollny 2 , have yielded the general results already indicated. 

 Macagno found that the greatest increase in the dry weight occurred in violet 

 light, but this was certainly either due to the presence of other rays or to the 

 absorption of external supplies of organic food. 



SECTION 61. Theoretical. 



No precise knowledge has hitherto been obtained of the chain of 

 processes by which the organic products of photo-synthetic assimilation are 

 produced, but it must be clearly borne in mind that the process is a purely 

 vital one. and hence in seeking a complete explanation we meet the same 

 difficulties as confront us when we are dealing with any vital phenomenon. 

 The fact that chloroplastids of unchanged external appearance may be 

 temporarily inactive shows that these special organs are capable of as- 

 similatory activity only when all the component parts co-operate in an 

 appropriate manner, and that the final result is produced not by a single 

 reaction but by the agency of a complicated and self- regulatory mechanism. 



The actual primary product of photosynthetic assimilation is unknown, 

 and we cannot say whether the first act in the assimilation of carbon 

 dioxide takes place in red or in blue-green chloroplasts in the same manner 

 as in green ones. The same end may often be attained by different means, 



1 Bert, Rech. s. 1. mouvem. d. 1. sensitive, 1870, p. 28 (From Me"m. d. 1'Acad. d. Bordeaux, 

 T. Vin); Compt. rend., 1870, T. i.xx, p. 338; 1871, T. LXXIII, p. 1444; Baudrimont, ibid., 1872, 

 T. LXXIV, p. 471 ; G. Kraus, Bot. Zeitung, 1876, p. 508. 



2 Hunt, Bot. Zeitung, 1851, p. 319; Sachs, ibid., 1864, p. 371, and Arb. d. Bot. Inst. in Wiirz- 

 burg, 1871, Bd. I, p. 56; Ad. Mayer, Versuchsst., 1867, Bd. IX, p. 396; R. Weber, ibid., 1875, Bd. 

 XVIII, p. 18; Morgen, Bot. Zeitung, 1877, p. 579; Wollny, Forsch. a. d. Geb. d. Agriculturphysik, 

 1894, Bd. xvil, p. 317 ; Macagno, Bot. Zeitung, 1874, p. 544. 



PFEFFER A a 



