34 



THE FOOD OF PLANTS 



disappear again when the latter is feeble or absent. Starch is therefore one 

 of the products of carbon dioxide assimilation, but it is quite uncertain 

 what are the precise changes which lead to its formation (Sect. 54), and 

 indeed no clear insight into the processes involved in the assimilation of 

 carbonic acid has as yet been obtained. It is not impossible that the 

 synthesis docs not always take place in precisely the same manner in all 

 plants, and it may be very different in purple bacteria and in normal 

 chlorophyllous plants. (Cf. Sect. 61.) 



The assimilation of carbon dioxide is a vital function, the chloroplastids 

 being living mechanisms specially adapted for this purpose, and capable 

 of exercising their functional activity only when all the essential component 

 parts are intact. The coloured pigment chlorophyll simply forms a co- 

 operating member of this mechanism, and indeed the assimilation of carbon 

 dioxide may be possible when no chlorophyll is present. Colourless chloro- 

 plastids, it is true, show no power of assimilating carbon dioxide, but the 

 yellow etiolin-corpuscles formed by most green plants when grown in 

 darkness may exhibit a faint power of carrying out this process. The full 

 assimilatory powers are, however, exhibited only by normal green chloro- 

 plastids *, and hence the importance of the chlorophyll pigment seems 

 merely to be due to the fact that by means of it a very pronounced 

 absorption of the radiant energy of light is possible. 



By means of the bacterium-method, Engelmann (Bot. Zeitung, 1881, p. 445) 

 observed in an isolated case that the etiolated chloroplastids of a plant grown 

 in darkness showed a faint evolution of oxygen when exposed to light. A possible 

 explanation, however, was that minute traces of chlorophyll were present, though 

 difficult of detection by direct observation, for as a matter of fact, many plants 

 form an abundance of chlorophyll in darkness. Ewart (Journ. of Linn. Soc., 1897, 

 Vol. xxxi, p. 573) has, however, conclusively proved that in the absence of all 

 traces of chlorophyll, etiolated chloroplastids may show a faint power of carbon 

 dioxide assimilation. After a prolonged sojourn in darkness this power is lost, 

 evidently owing to some functional derangement, since the same amount of etiolin 

 may be present as before. Etiolin (xanthophyll) is therefore an assimilatory 

 pigment, though not nearly so efficient as is chlorophyll. Kohl (Ber. d. Bot. Ges., 

 Bd. xv, 1897, p. in) supposes that the secondary assimilatory maximum in the 

 blue region of the spectrum is due to the absorptive activity of the xanthophyll and 

 carotin present in all chloroplastids. 



Chlorophyll when isolated is as little able to effect any assimilation of 

 carbon dioxide 2 as when it forms part of a dead chloroplastid, and it is 



1 Cf. Boussingault, Ann. d. sci. nat., 1864, v. sdr., T. i, p. 315. and 1869, v. sen, T. x, p. 337 ; 

 Engelmann, Bot. Zeitung, 1887, ? 4 J 9- 



2 Regnault's contradictory results are incorrect (Compt. rend., 1885, T. ci, p. 1293"). See Jodin 



