CARBOHYDRATES. 53 



which is present in plants; and in fact, the chlorophyll is not deposited free 

 in the tissues of these animals, but the chlorophyll-bearers are those algae 

 which exist together with the animal cell and have a commensal existence. 

 This sort of parasitism is met with very frequently, for instance in 

 lichens, 1 which are composed of fungi and algae. Again, we find the same 

 sort of symbiosis in higher organisms. Undoubtedly, for example, the 

 bacteria found in the alimentary canal illustrate this relation; they serve, 

 as we shall see later on, to convert carbohydrates (cellulose, etc.) into a 

 form capable of absorption. 



By means of carbonic acid alone, the cell cannot build up organic sub- 

 stances; hydrogen, which forms an integral component of all the com- 

 plicated compounds of the carbohydrate series which the plant organism 

 produces, is lacking. Hydrogen is obtained by the plant chiefly from 

 water in the soil. Herein the plant finds another source of oxygen. The 

 way in which the plant cells utilize, by the help of chlorophyll and the 

 sun's rays, this carbonic acid and water, or, in other words, the first 

 products formed, is still a matter of conjecture. We know merely that 

 all syntheses start with a reduction process; oxygen is set free. In fact, 

 we can follow the assimilation of carbonic acid either by determining the 

 loss of carbonic acid in a gas mixture, or by determining the oxygen 

 evolved. The latter can be observed directly by immersing the parts of 

 a plant in water. Engelmann 2 ingeniously made use of bacteria as a 

 reagent for oxygen. If, for example, a thread of algae and some aerobic 

 bacteria are placed under an air-tight cover-glass, it will be noticed at 

 first that the latter are very lively. If, now, the preparation is kept in the 

 dark, the motion of these bacteria will cease altogether after a time, show- 

 ing that all the oxygen has been consumed. Then, on bringing the prep- 

 aration to the light, the bacteria begin to become most active because the 

 thread of algae has begun to assimilate carbon dioxide and water with 

 elimination of oxygen. By means of this very sensitive test as little as 

 one billionth part of a milligram of oxygen may be detected. Engelmann, 

 as already stated, has by means of this method studied the different parts 

 of the solar spectrum, testing it with regard to its effect upon plant assim- 

 ilation. Beyerinck 3 later in a similar way made use of the luminescence 

 of certain bacteria, which also depends upon the presence of free oxygen, 

 for the detection of carbonic-acid assimilation. 



With regard to the synthetical products first formed by the plant, 



1 Schwendener: Nagelis Beitrage z. wissensch. Botanik H. 2, 3, and 4, Leipsic, 1860- 

 1868. -de Bary: "Die Erscheinung der Symbiose," Strassburg, 1879. O. Hertwig: 

 "Die Symbiose oder das Genossenschaftsleben in Tierreich," Jena, 1883. 



2 Bot. Ztg. loc. cit. 



3 Ibid. (1890) 744. See also Hans Molisch: "Die Lichtenwicklung in den Pflan- 

 zen, Naturwissenschaftliche Rundschau, 20, 505 (1905). 



