104 



METABOLISM 



The decomposition of carbon-dioxide shows itself most obviously in the 

 giving off of oxygen. The peculiar characteristics of water plants furnish us 

 with a method of demonstration of this phenomenon equally serviceable for 

 lecture purposes and for laboratory work. Fix a branch of Elodea Canadensis, 

 or a submerged leaf of Potamogeton, to a glass rod in the manner shown in 

 Fig. 22, and arrange that the branch or leaf while under water is illuminated 

 by sunlight or artificial light. Presently we shall see issuing in regular succes- 

 sion from the cut end a series of small air-bubbles. This stream of bubbles 

 may be accounted for quite simply. If we assume that the plant has been in 

 the dark for some time it will have accumulated in its intercellular spaces, in 

 addition to oxygen and nitrogen, a certain amount of carbon-dioxide. If this 

 last be decomposed, the oxygen formed, in accordance with the known characters 

 of gases, will occupy the same space as the carbon-dioxide did ; and hence there 

 would appear no reason why air should escape from the cut 

 surface. This will occur when the disappearing carbon- dioxide 

 is replaced by diffusion from without, thus producing an 

 excess pressure in the intercellular spaces of the plant. This 

 excess pressure remains constant so long as carbon-dioxide is 

 present outside the plant and undergoes decomposition within it. 

 If we arrange an apparatus with the object of collecting the air 

 which streams from several submerged branches exposed simul- 

 taneously to light, e. g. by covering them with a test-tube 

 filled with water, we shall be able to study the gas much more 

 conveniently and accurately. That the gas consists largely of 

 oxygen is proved by the fact that it causes a glowing splinter 

 of wood to burst into flame, and more exact analysis establishes 

 the fact that it never consists of pure oxygen, but always has 

 a demonstrable admixture of nitrogen. This is only to be ex- 

 pected, since, if the intercellular spaces of the plants have been, 

 owing to the decomposition of carbon-dioxide, rendered richer 

 in oxygen than the surrounding water, nitrogen must pass into 

 the intercellular spaces from the water. Further, each individual 

 air-bubble in its passage through the water, and finally the 

 whole amount of gas present, must receive additions of nitrogen 

 by diffusion so long as the experiment continues. No doubt 

 we could arrange an experiment in such a way that pure oxygen 

 would be obtained, that is to say by removing the nitrogen 

 originally held in solution by the water, taking care to replace 

 the carbon-dioxide lost in the process, and by preventing the entrance of 

 fresh supplies of nitrogen. The form of the first experiment suggests 

 several criticisms ; for instance, the stream of air-bubbles, at least at first, 

 might be due to expansion of the air in the intercellular spaces by 

 heating, and later on, to some extent, by thermo-diffusion. Evidence, how- 

 ever, that the stream of air-bubbles is entirely dependent on the presence of 

 carbon-dioxide in the water disposes of all these objections. The addition of a 

 small quantity of lime-water and the consequent precipitation of the carbonic 

 acid (F. SCHWARZ, 1881), or the use of freshly boiled water, is sufficient to bring 

 the stream of air-bubbles at once to a standstill, without doing any damage to 

 the plant. 



The method just described is not only qualitative but may be made quanti- 

 tative as well, since the number of bubbles which come off in a unit of time 

 from a given specimen forms a means of measuring the amount of carbon- 

 dioxide decomposed. It is true we must not compare several specimens with 

 each other off-hand, for a vigorously active branch may give off few but large 

 air-bubbles, while a weakly assimilating one may furnish many small ones. The 



Fig. 22. From 

 DETMER'S Prac- 

 tical Plant Physi- 

 ology. 



