348 THE FOOD OF PLANTS 



of iodine in carbon bi-sulphide, that is in the dark infra-red rays, the 

 same amount of carbon dioxide was produced as in darkness. 



Only a small portion of the energy of the light falling upon a leaf is 

 utilized in decomposing carbon dioxide. Thus a square metre of actively 

 assimilating leaf surface of Nereum oleander forms 0-000535 grm. of starch 

 in one second, by which an amount of energy equivalent to 2-2 caloric units 

 is obtained, which is less than i per cent, of the total energy of sunlight 

 received, according to Pouillet, by this surface area on a bright sunny day 

 in one second, which is 333 heat units'. Similarly, Detlefsen 2 found that 

 when a leaf was prevented from assimilating by the absence of carbon 

 dioxide, a thermopile placed behind it registered a slight rise in temperature, 

 but only so much as to indicate that the leaf when assimilating utilized at 

 most not more than 0-3 to i-i per cent, of the total energy of the incident 

 light. 



The fact that chlorophyll continues to absorb light in non-assimilating 

 as well as in dead leaves suffices to show that the total energy of the 

 absorbed light is not used in assimilation, as Engelmann apparently 

 concluded to be the case from the proportionality existing between 

 absorption and assimilation. A similar relationship is exhibited in other 

 photo-chemical processes in which only a portion of the absorbed energy is 

 utilized. Nor does it follow that the energy of the light rays is directly 

 employed in the decomposition of carbon dioxide, although in all probability 

 this is actually the case. Nevertheless, this is merely a hypothetical 

 assumption, for the specially active rays might merely exert a stimulating 

 action upon the process of assimilation, the necessary energy being derived 

 from the heat-rays directly absorbed, and perhaps also from the heat 

 vibrations induced by the absorption of more rapidly vibrating light-rays 3 . 



The part played by chlorophyll in absorbing and rendering available 

 the energy of light is certainly of great importance, but assimilation is 

 possible only when the different parts of the assimilatory mechanism all 

 co-operate together in the proper manner, for just as a locomotive refuses 

 to work when its machinery is out of order, however high the steam pressure 

 may be, so also may chloroplastids, rendered entirely or partially inactive 

 by some invisible internal alteration, be wholly or partly incapable of 



1 Pfeffer, Bot. Zeitung, 1873, p. 429; Ostwald, Lehrb. d. allgem. Chemie, 1893, 2. Aufl., 

 Bd. n, i, p. 1070. On radiation from the sun, see also Rubner, Centralbl. f. Physiol., 1895, 

 Bd. VIH, p. 664. [One gramme of starch when burnt produces 4,100 caloric units.] 



3 Detlefsen, Arb. d. BoL Inst. in Wiirzburg, 1888, Bd. in, p. 543. The experiments are not 

 quite faultless. N. J. C. Miiller's comparative experiments (Bot. Unters., 1872, Bd. I, p. 339) with 

 dead and living leaves are without value, owing to the changes in the absorptive powers of a leaf 

 which are coincident with death. Cf. Reinke, Bot. Zeitung, 1881, p. 209. 



8 Engelmann, Bot. Zeitung, 1884, p. 102; 1886, p. 68 ; 1888, p. 689; Ostwald, I.e., pp. 1056, 

 1087. Cf. Koppen, Warme u. Pflanzenwachsthum , 1870, p. 6.} ; Mayer, Lehrb. d. Agriculturchemic, 

 1871, p. 31 ; Pfefier, Energetik, 1892, p. 204. 



