204 



THE MECHANISM OF GASEOUS EXCHANGE 



carbon dioxide. r] he continuance of this process causes a more or less rapid stream 

 of bubbles to emerge from the cut stem of illuminated submerged plants with well- 

 developed intercellular systems (Elodea, Myriophyllum, Ceratophyllum, &c.) (Fig. 25), 

 and the number of these bubbles affords a measure of the amount of carbon dioxide 

 assimilated (Sect. 52). That the stream of bubbles is due to the production of 

 oxygen is shown by the following facts: (i) it rises and falls within certain limits 

 as the intensity of the illumination increases or diminishes, and ceases almost imme- 

 diately in darkness ; (2) it stops without the plant being injured when all free carbon 

 dioxide is removed by the addition of lime-water \ Similarly, insolation produces 

 no continuous stream of bubbles unless the function of photosynthetic assimilation 

 can be exercised 2 , although a rise of temperature may cause a few bubbles to escape 



for a moment or two. It is, however, always possible 

 that by a rise of temperature under certain conditions a 

 stream of bubbles may be produced in quite a different 

 way, for in the phenomenon of thermo-diffusion dis- 

 covered by Dufour and studied by Feddersen 3 a current 

 of gas may flow from the warmer to the colder side of 

 a separating membrane. Moreover, in a closed porous 

 clay cell or bladder a distinct pressure, increasing as the 

 temperature rises, is produced when the enclosed air is 

 saturated with moisture, but the surrounding air remains 

 dry 4 . Similarly, when a plant containing air rich in 

 oxygen is brought into water rich in carbon dioxide, it 

 is easy to see why a stream of bubbles may continue to 

 escape for a time in darkness 5 . It is also evident that the 

 bubbles, under normal conditions, will not be composed 

 of pure oxygen, but that their composition will depend 

 upon the intensity of the production of oxygen, upon 

 the percentage amounts of gas dissolved in the water, 

 as well as upon many other factors. 



It is true that the production of oxygen rarely 

 gives rise to any very great pressure in intact acquatic 

 plants, but nevertheless it is sufficient to force out 

 bubbles of gas against a water-pressure of 20 to 30 cm., if the cut surface of the 

 stem is at that depth below the surface when the plant is upright fl . No detailed 

 investigations have as yet been made as to the manner in which the high internal 

 gaseous pressures found in the Fucaceae, &c. are produced (Sect. 29). 



FlG. 25. The glass rod (b), to 

 which the plant is attached by a 

 thread, passes through the cork (a), 

 by whicn it is held firmly. 



' F. Schwarz, Unters. a. d. Bot. Inst. z. Tubingen, 1881, Bd. I, p. 97. 



2 An exception to the contrary is given by N. J. C. Miiller, Bot. Unters., 1876, Bd. I, p. 380. 



8 Details by Naumann, Allgem. Chem., 1877, p. 261. 



4 For an explanation of this phenomenon, see Kundt, Ann d. Physik n. Chemie, 1877, N. F., 

 Bd. n, p. 17. 



5 Van Tieghem, Ann. d. sci. nat., 1868, v. ser., T. IX, p. 269; Lecoq, Compt. rend., 1867, 

 T. LXV, p. 1114, and 1869, Bd. LXIX, p. 531 ; N. J. C. Miiller, Jahrb. f. wiss. Bot., 1873-4, Bd. IX, 

 p. 37; Devaux, Ann. d. sci. nat., 1889, vii. ser., T. ix, p. 138. 



6 Lechartier, Ann. d. sci. nat., 1867, v. ser., T. vm, p. 364. 



