338 THE FOOD OF PLANTS 



taking the respiratory activity into consideration, as was done by Kreusler ; 

 hence the earlier researches do not give the true optimum or the actual 

 relationship between assimilation and temperature l . 



The assimilatory curve is not the same in all plants, for it is evident 

 that the algae of a snow-flora must be able to assimilate carbon dioxide at 

 very low temperatures, since they may pass through their entire life-history 

 at a temperature which hardly ever rises much above zero 2 . Active and 

 resistant chloroplastids are actually capable of assimilating carbon dioxide 

 for a time at zero or even a few degrees below it, though when the exposure 

 is more prolonged this power is gradually lost even in comparatively resistant 

 plants. Ewart has shown that the immediate effect of a low temperature 

 is sufficient to cause all evolution of oxygen to cease at from 4 C. to 8 C. in 

 tropical plants, and at from o C. to 2 C. in warm-temperate, sub-tropical, and 

 water-plants, while in cool-temperate, arctic and alpine forms, assimilation 

 may cease only when the plants are completely frozen :t . 



Amount of water. A slight diminution of turgidity sufficient to close the 

 stomata will render the absorption of carbon dioxide extremely difficult, and 

 hence may markedly diminish the assimilatory activity of a sub-aerial leaf, 

 whereas in lichens and in the leaves of mosses, where stomata are absent, 

 such a slight decrease produces but little effect upon the assimilation of 

 carbon dioxide, and this process ceases only when the loss of water is very 

 great 4 . Similarly, dilute saline solutions seem to decrease the power of 

 assimilation but little, provided that they exert no poisonous influence upon 

 the chloroplastids, and even a plasmolysed cell may be able to assimilate 

 more or less actively. A solution of cane-sugar of over 40 per cent, strength 

 completely arrests the assimilation of carbon dioxide, and prolonged immer- 

 sion in a 30 per cent, to 35 per cent, solution of cane-sugar may render 

 the chloroplastids of Elodea, Catharinea, Bryum, Orthotrichum temporarily 



1 Kreusler, Landw. Jahrb., 1887, Bd. XVI, p. 711 ; 1888, Bd. xvit, p. 161 ; 1890, Bd. XIX, 

 p. 649; Heinrich, Versuchsst., 1881, Bd. XIII, p. 136; Bbhm, Sitzungsb. d. Wien. Akad., 1873, 

 Bd. I.XYII, Abth. i; Schutzenberger u. Quinquaud, Compt. rend., 1873, T. LXXVII, p. 272; 

 Prianischnikow, Bot. Jahrb., 1876, p. 897. No importance can be attached to Fauncopret's attempt 

 (Compt. rend., 1864, T. LVill, p. 334) to represent the relation between assimilation and tempera- 

 ture by a simple equation. 



a Cf. Ewart, Annals of Botany, 1898, Vol. XII, p. 377. 



* Boussingault, Ann. d. sci. nat., 1869, v. se"r., T. X, p. 336; Kreusler, I.e., 1888, p. 163; 

 Ewart, 1. c., pp. 389, &c. [Jumelle observed that a few conifers and lichens were able apparently 

 to feebly assimilate CO 2 at 4OC. (Rev. ge"n.. 1892, T. IV, p. 263). Since, however, all respira- 

 tion ceases at ioC. to i2C.,it is manifestly impossible that any assimilation of carbon dioxide 

 can take place at 4OC., for CO 3 -assimilation is a vital process involving protoplasmic activity. 

 Ewart has, indeed, shown that the changes in the composition of the air of the receiver noticed 

 by Jumelle do not necessarily indicate that any carbon dioxide had been assimilated at the 

 temperature in question (Ewart, I.e., p. 402).] 



* Cf. Sect. 58. Gasometric experiments by Boussingault, Agron., &c., 1868, T. iv, p. 317 ; 

 Kreusler, Landw. Jahrb., 1885, Bd. xiv, p. 951. Mosses, &c. : Jumelle, Rev. ge"n. d. Bot., 1892, 

 T. iv, pp. 168, 318; Bastit, ibid., 1891, T. Ill, p. 521. 



