INFLUENCE OF EXTERNAL CONDITIONS ON CO^ASSIMILATION 333 



SECTION 58. The Influence of External Conditions on 

 the Assimilation of Carbon Dioxide. 



The assimilatory activity being a vital process, is liable to be influenced 

 in various ways by the external conditions, even although the illumination and 

 the supply of carbonic acid gas remain constant. Indeed the formation and 

 development of the chloroplastids themselves are possible only when certain 

 essential conditions are fulfilled, while when adult their assimilatory powers 

 may be directly or indirectly modified or altered. In investigating such 

 alterations it must be remembered that various other changes may occur at 

 the same time, and that the prolonged action of any agency may ultimately 

 produce a different effect to that caused by it when first applied (cf. 

 Chap. I). It is, however, only possible to give a very condensed account 

 of these problems, as well as of the conditions necessary for the develop- 

 ment of the chloroplastids. 



Conditions necessary for the development of the chloroplastids. It is the 

 inherent nature of the embryonic chromatophore which determines in 

 what direction it will develop, and whether it has a tendency to become 

 a chloroplastid (Sect. 53). The latter turns green, as a general rule, only 

 when exposed to light, for in darkness yellow etiolin corpuscles are formed, 

 and these even in weak light rapidly produce chlorophyll and turn green, 

 provided that they have not lost this power through being kept too long 

 in darkness *. Certain plants, however, form green and functionally active 

 chloroplasts in the absence of light, as is the case in the cotyledons of seedlings 

 of various Coniferae (Pinus sylvestris, picea, &c.), although the winter buds 

 of these plants and the seedlings of Gingko biloba form no chlorophyll in 

 darkness, and seedlings of Larix and Thuja turn green in parts only 2 . 



The formation of chlorophyll is therefore not necessarily connected 

 with the presence of light, and its non- formation in darkness may be due 

 to disturbances of nutrition or to pathological conditions, for the chloro- 

 plastids of many plants become pale or discoloured after a few days' 

 darkness, whereas those of others, such as plants belonging to the Cactaceae 



1 Literature: Sachs, Flora, 1862, p. 162, and Bot. Zeitung, 1862, p. 366; Wiesner, Entstehung 

 d. Chlorophylls, 1877, p. 86 ; Batalin, Bot. Zeitung, 1871, p. 677 ; Monteverde, Acta Horti Petro- 

 politani, 1894, Bd. xni, p. 215. On intermittent illumination, cf. Mikosch u. Stohr, Sitzungsb. d. 

 Wien. Akad., 1880, Bd. LXXXII, Abth. i, p. 629. On final loss of power of turning green, Detmer, 

 Landw. Jahrb., 1882, Bd. xi, p. 224. See also Ewart, Joum. Linn. Soc. Bot., Vol. XXXI, 1896, 

 pp. 560, &c. 



2 Sachs, Lotos, 1859, and Flora., 1864, P 55 5 Mohl > Bot - Zeitung, 1861, p. 258. Schmidt's 

 objections (Einige Wirkungen d. Lichtes auf Pflanzen, 1870, p. 18) are without value. In Ferns, 

 Mosses, Chara, &c., light does not seem to be essential for the formation of chlorophyll. See 

 Schimper, Jahrb. f.wiss. Bot., 1885, Bd. xvi, p. 159, and the literature there given ; also Ewart, I.e., 

 pp. 555, 563, &c. ; Frank, Die natiirliche wagrechte Richtung von Pflanzentheilen, 1870, p. 27; 

 Molisch. Oest. Bot. Zeitschr., 1889, Nr. 3; Wiesner, Entstehung d. Chlorophylls, 1877, p. 117. 



