THE SOURCES OF CARBON DIOXIDE 331 



dioxide from Calcium bicarbonate (Ca 2 (HCO 3 ), but can also make use 

 of 70 per cent, of the carbon dioxide contained in sodium bicarbonate 1 

 (Na HCO 3 ). 



When the stomata of land-plants are occluded by capillary water, 

 the rate of gaseous exchange undergoes a very marked diminution, and 

 hence sub-aerial leaves wetted by water assimilate but little carbonic acid 

 gas when immersed, although so long as the adherence of an intermediary 

 film of air is shown by their silvery appearance, rapid gaseous exchange 

 is still possible, and hence such leaves may continue to assimilate fairly 

 actively under water 2 (Sect. 29). 



The bubbles of gas evolved from assimilating aquatic plants (Sect. 32) are not 

 pure oxygen but contain larger or smaller amounts of nitrogen and carbon dioxide, 

 the amount of oxygen varying from 25 to as much as 98 per cent.'', when the 

 evolution has continued for some time and no fresh nitrogen has been allowed 

 access to the plant (Sect. 54). The number of bubbles also depends upon the 

 diosmosis of oxygen into the intercellular spaces, where it accumulates together 

 with other gases, and hence the evolution of gas bubbles does not form an 

 accurate indication of the actual amount of oxygen produced. Indeed, when but 

 little oxygen is formed, it may entirely dissolve in the surrounding water without 

 any bubbles appearing. Nevertheless an approximate measure of the activity of 

 assimilation may be obtained in this manner, and the accuracy of the method 

 largely depends upon the way in which it is applied 4 . 



The activity of assimilation is increased by the addition of carbon dioxide 

 to the air, and hence it follows that a well illuminated chloroplastid does not 

 normally obtain as much of this gas as it is capable of decomposing. An 

 excess of carbon dioxide exerts a poisonous influence upon the plant , 

 and hence there is a certain optimal percentage at which assimilation is 

 most active, and this varies not only in different plants, but also according to 

 the external conditions, for both the intensity of the illumination and the 

 rapidity of gaseous exchange must influence the assimilatory curve. Thus 

 when the stomata are closed, assimilation may be active only when the 

 percentage of carbon dioxide present in the external air is abnormally 

 high, whereas under normal conditions a slight increase in the percentage 



1 Hassak, Unters. a. d. Bot. Inst. z. Tubingen, 1888, Bd. II, p. 471 ; Weyl, Sitzungsb. d. 

 Phys.-Med. Ges. in Erlangen, 1881, i. Aug. Cf. Sect. 23. 



2 Bohm, Sitzungsb. d. Wien. Akad., 1872, Bd. LVI, Abth. i, p. 169 ; Nagamatsz, Arb. d. Bot. 

 Inst. in Wiirzburg, 1887, Bd. in, p. 389. [Bonnier (Compt. rend., cxxvi, 1898, p. 1001) has 

 actually succeeded in developing plants of Mimosa pudica under water.] 



3 See de Candolle., Pflanzenphysiol., 1833, Bd. I, p. 102 ; Daubeny, Phil. Trans., 1836, Pt. i, 

 p. 157 ; Cloez et Gratiolet, Ann. d. chim. et d. phys., 1851, iii. seV., T. xxxii, p. 51 ; N. J. C. 

 Muller, Jahrb. f. wiss. Bot., 1867-8, Bd. VI, pp. 484, &c. 



4 Cf. Pfeffer, Arb. d. Bot. Inst. in Wiirzburg, 1871, Bd. I, p. 52 ; Reinke, Bot. Zeitung, 1884, p. 24. 



5 Grischow, Unters. Uber d. Athmung, 1819, p. 33 5 Boussingault, Agron., &c., 1868, T. IV, 

 pp. 286, &c. 



