334 THE FOOD OF PLANTS 



and Coniferae, as well as those of Elodea, Chara, &c., are more resistant and 

 may remain green for more than a month in darkness T . When the supply 

 of nutriment becomes deficient colourless plastids are formed instead of 

 etiolin corpuscles, but most seedlings and bulbous plants are able in dark- 

 ness to form yellow etiolin corpuscles which exhibit a faint but distinct power 

 , of assimilating carbon dioxide (HcliantJius, Cncurbita, Allium, Beta, &c.). 

 In other cases, as in Zca Mays, the etiolin corpuscles, though deep yellow 

 in colour, have no such power, and in all cases this power of assimilation 

 is lost in continued darkness after a shorter or longer period 2 . 



Plants turn green most rapidly in light of moderate intensity, and 

 various observations indicate that in the living chloroplastid the decomposi- 

 tion and reconstruction of chlorophyll continue without cessation, especially 

 in strong light ^. No direct conclusion as to the behaviour of chlorophyll 

 under normal conditions can be drawn from the blanching of solutions 

 of chlorophyll, or of chloroplastids when exposed to intense sunlight. 

 Pringsheim was able to completely destroy the chlorophyll in chloro- 

 plastids by means of concentrated sunlight, but the fact that they remained 

 colourless showed that the power of regenerating chlorophyll had also been 

 lost, if it previously existed 4 . A single day's exposure to full sunlight may, 

 however, cause the leaves of Fisonia alba and of some species of Selaginella 

 to lose all green colour and become pale yellow or almost white. The chloro- 

 plastids, however, retain the power of starch- formation, and in feeble light the 

 green colour may gradually be restored. Various other plants of a smaller 

 degree of sensitiveness show similar but less marked reactions to sunlight 5 . 



All the visible rays of the spectrum may excite the formation of 

 chlorophyll, but according to Reinke the maximal effect is produced by 

 those between Fraunhofer's lines B and D, while Wiesner has shown that 

 in strong light the more refrangible rays are most effective* 5 . 



1 Literature: Sachs, Flora, 1862, p. 218; Hot. Zcitting, 1864, p. 290; Wiesner, Oberdie Beziehung 

 d. Lichtes z. Chlorophyll, 1874, p. 49 (Sep.-abdr. a. Sitzungsb. d.Wien. Akad., Bd. LXIX, Abth. i) ; 

 Frank, Bot. Centralbl., 1882, Bd. X, p. 230; Reinke, Sitzungsb. d. Berl. Akad., 1893, p. 528; 

 Ewart, Journ. Linn. Soc., Vol. XXXI, 1896, pp. 564, 573. 



2 Ewart, Journ. Linn. Soc. Bot., 1896, Vol. XXXI, pp. 556, 561, 564, &c. 



3 Sachs, Flora, 1862, p. 214 ; Famintzin, Jahrb. f. wiss. Bot., 1867-8, Bd. VI, p. 47 ; Wiesner, 

 Die natiirlichen Einrichtungen zum Schutze d. Chlorophylls, 1876, p. 21 v Sep.-abdr. d. Festschr. d. 

 Zool.-Bot. Ges. in Wien); Ilaberlandt, Unters. iiber die Winterfarbung ausd. Blatter, 1876, p. 10 

 (Sep.-abdr. a. d. Sitzungsb. d. Wim. Akad., Bd. LXXII, Abth. i) ; Schimper, Jahrb. f. wiss. Bot., 

 1885, Bd. XVI, p. 165 ; Reinke, Bot. Zeitung, 1885, p. 133. Ewart (Annals of Botany, Vol. xil, 

 1898, p. 392) has calculated that a chloroplastid of Elodea may produce, during a day's exposure to 

 bright light, as much as four to five times the amount of chlorophyll it originally contained. 



* Pringsheim, Jahrb. f. wiss. Bot., 1879-81, Bd. xil, p. 345. Even after a year no regeneration 

 of chlorophyll took place in chloroplastids of Chara decolorized by prolonged exposure to sunlight 

 (Ewart, Journ. Linn. Soc., Vol. XXXI, p. 573), whereas in Elodea they may become green again 

 (Ewart, Annals of Botany, 1898, Vol. XII, p. 392). 



s Ewart, Annals of Botany, 1897, Vol. XI, p. 442 ; Wiesner, Sitzungsb. d. kais. Akad. d. Wiss. 

 in Wien, 1894. p. 305. 



6 Reinke. Sitznngsb. d. Berl. Akad., 1893, p. 536. The observations o r Gardiner and fluillemin 



