INFLUENCE OF EXTERNAL CONDITIONS ON CO..-ASSIMILATION 335 



Temperature. A low temperature at which slow growth is possible 

 is often insufficient for the formation of chlorophyll, so that, as Sachs 

 observed, plants developed at the minimal temperature for growth do not 

 turn green, although the chloroplastids become differentiated, and, according 

 to Elfving, light causes them to become a deeper yellow 1 . The winter 

 browning of Conifers and other plants is caused by the combined action of 

 light and a low temperature, and is due to a partial decomposition of the 

 chlorophyll accompanied by a formation of brown, yellow, or red pigments, 

 and frequently by a partial disorganization of the chloroplastids. In some 

 cases the browning takes place only when the chlorophyll-containing cells 

 are fatally affected, as in Ilex, &c., but in most Conifers the chloroplastids 

 may recover, turn green, and become functionally active when restored to 

 normal conditions of temperature 2 . 



Oxygen : Food supply. The formation of chlorophyll seems to be in 

 general comparatively easily inhibited, for Correns 3 has shown that a partial 

 pressure of oxygen, at which growth and heliotropic curvature are still 

 possible, does not suffice for the formation of chlorophyll. Unfavourable 

 nutritive conditions may frequently prevent the chloroplastids partially 

 or entirely from turning green, and this is perhaps why the absence 

 of iron produces chlorosis and prevents the normal differentiation of the 

 chloroplastids (Sect. 74). 



On the other hand, a self-regulatory development of chlorophyll occurs 

 when a stem of Cuscuta becomes a deeper green if deprived of a host- 

 plant 4 , or when a shoot grown from a colourless root develops green 

 chloroplastids on exposure to light. 



Permanent and transitory inhibition. The chloroplastids frequently 

 lose their assimilatory function in the normal progress of development, as 

 when they become converted into red or brown chromoplasts in a ripening 

 fruit, or when the chlorophyll disappears as the leaves acquire their autumnal 



are given by Reinke on p. 529. Sachs, Bot. Zeitung, 1864, P- 353 > Wiesner, Entstehung d. Chloro- 

 phylls, 1877, P- 39! Die heliotrop. Erscheinungen im Pflanzenreich, 1878, p. 194, footnote. 



1 Sachs, Flora, 1864, p. 497. Wiesner has shown that there is an optimum temperature for the 

 formation of chlorophyll (Entstehung d. Chlorophylls, 1877, p. 95). Elfving, Arb. d. Bot. Inst. in 

 Wiirzburg, 1880, Bd. I, p. 495. 



2 Mohl, Vermischte Schriften, 1845, p. 375. Details by Schimper, Jahrb. f. wiss. Bot., 1885, 

 Bd. xvi, p. 166; G. Haberlandt, Uber die Winterfa'rbung ausdauernder Blatter, 1876, p. 10 

 (Sitzungsb. d. Wien. Akad., Bd. Lxxil, Abth. i) ; Ewart, Journ. Linn. Soc., 1896, Vol. xxxi, p. 390. 

 [H. Winkler (Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 538) has shown that such discoloured chloro- 

 plastids may acquire the power of starch formation (if supplied with sugar) before they turn green 

 or become capable of carbon dioxide assimilation.] 



3 Correns, Flora, 1892, p. 14. On the retarding influence of an accumulation of carbon dioxide 

 on the formation of chlorophyll cf. Bohm, Sitzungsb. d. Wien. Akad., 1873, p. 14. [Microscopical 

 preparations of etiolated leaves of Elotiea, if thinly ringed with vaseline, may show slow rotation, 

 but are unable to turn green. Cf. Ewart, Journ. Linn. Soc., 1896, Vol. XXXI, p. 566.] 



4 Pierce, Annals of Botany, 1894, Vol. via, p. 81. On the influence of the food-supply on the 

 turning green of seedlings, see Palladin, Ber. d. Bot. Ges., 1891, p. 229. 



