THE FOOD OP PLANTS. 35 



few wheat seedlings grown in the dark, and fix the vessels by 

 means of a suitable stand with their mouths under mercury (see 

 Fig. 11). We replace the water in a up to a certain point with 

 Hydrogen, that in b by atmospheric air. If the vessels are 

 now exposed to the influence of diffuse light, the seedlings in b 

 rapidly become green, those in a do not. 3 



It may be remarked that the Hydrogen is prepared by the 

 action of zinc on dilute Hydrochloric acid, and purified by passage 

 through solutions of potash, Silver nitrate, and Potassium per- 

 manganate. The gas may be developed in a Kipp's apparatus 

 (see Section III.). 



1 See Wiesner, Sitzunjsberickte d. Akad. d, Wiss. in Wien, Bd. 69, I. 

 Abtheilung. 



2 See Wiesner, Die Entstehung des Chlorophylls in den Pflanzen, Wien, 1877. 



3 See Detmer, Landicirtscha/tl. Jahrbiicher, Bd. 11. 



11. The Production of Oxygen in Assimilation. 



Under the influence of light decomposition of Carbon dioxide 

 takes place in the chlorophyll grains, Oxygen is set free, and 

 with suitable research material can readily be detected as such. 



We place in a glass say 200 c.c. of spring water, into which 

 we lead not too much pure Carbon dioxide. The Carbon dioxide 

 is prepared from marble by treatment with dilute Hydrochloric 

 acid, and, before passage into the spring water, must be freed 

 from any Hydrochloric acid carried over, by passing through a 

 solution of Sodium bicarbonate. We first select for experiment 

 a fairly long shoot end of Hippuris vulgaris, which, as shown in 

 Fig. 12, is immersed below water with its base passing into a 

 test-tube filled with water. The arrangement is now exposed for 

 some time to direct sunlight. It is found that bubbles of gas 

 spring from the cut end of the shoot, and gradually a considerable 

 quantity of gas collects in the test-tube. If we close the mouth 

 of the test-tube under water with the finger, invert it, and 

 introduce a glowing splinter of wood, it at once bursts into bright 

 flame. Our plant has therefore produced Oxygen. 



This extremely interesting experiment for demonstrating the 

 liberation of Oxygen by assimilating plants may also be made in 

 a somewhat different way (see Fig. 13). A large number of 

 shoots of Elodea or Ceratophyllum are placed under a funnel in a 



