THE FOOD OF PLANTS. 39 



we experiment with threads of Spirogyra, the aggregation follows 

 the course of the green bands. Our aerotropic swarmers thus 

 form an excellent reagent for the presence of Oxygen. When the 

 alga in the preparation is not illuminated, the movement of the 

 bacteria ceases ; with renewed illumination it at once recom- 

 mences, since then Oxygen is once more set free in adequate 

 quantity through assimilation.* 



1 An accurate account of the influence of rays differing in refrangibility on 

 the process of assimilation will be found in my LehrbuchderPjianzenphysiologie. 

 On assimilation experiments in the objective spectrum, see Pfeffer, Botan. 

 Z titling, 1872, No. 23. As to the arrangements for the objective spectrum 

 see the section on Heliotropic Nutations. 



2 See Heinrich, Versuchsstationen, Bd. 13, p. 136. 



8 See Detmer, Landwirthschaftl. Jahrbiicher, Bd. 11. 

 4 See Eugelmanu, Hotau. Zeituny, 1881 and 1882. 



12. Carbon Dioxide and Assimilation. 



The whole of the Carbon dioxide which is worked up in assimi- 

 lation is derived ultimately from the atmospheric air. This is a 

 mixture of gases, which, apart from a few non-essential consti- 

 tuents, consists of about 79 per cent, by volume of Nitrogen, 

 Ul per cent, by volume of Oxygen, and a small quantity of 

 Carbon dioxide (in 10,000 volumes of air only 3 volumes of 

 Carbon dioxide are present). That the air contains Oxygen may 

 easily be demonstrated. We fasten a piece of cotton wool soaked 

 in alcohol at the end of a thick wire bent twice at right angles. 

 The alcohol is ignited and an inverted glass cylinder is placed 

 over the flame. If we now quickly lower the mouth of the 

 cylinder under water, the fluid at once rises in it, while the flame 

 quickly goes out. The alcohol requires Oxygen for its combustion, 

 and takes it from the air in the cylinder. As the Oxygen dis- 

 appears, the water rises in the cylinder. 



That atmospheric air contains Carbon dioxide is also easy to 

 demonstrate. For this purpose a stream of air is led through 

 clear baryta water by means of a water-air pump or a drop 

 aspirator. The baryta water gradually becomes cloudy since the 

 Carbon dioxide causes a precipitation of Barium carbonate. In 



* Engelmann's method has also been employed to measure the intensity of 

 Carbon dioxide decomposition and Oxygen production in different parts of the 

 spectrum. For this purpose the mikrospectral objective of Zeiss is suitable. 



