WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 189 



(d) To show that oxygen is necessary for the formation 

 of chlorophyll, fill a test-tube with mercury, or with boiled 

 and cooled water, invert it in water, and pass under its rim 

 some etiolated Mustard seedlings. Though exposed to 

 light, the seedlings do not become green, owing to lack of 

 oxygen. Another method is to place heavier seedlings 

 e.g. Bean, Pea in a glass jar and cover it with water. In 

 each case similar etiolated seedlings should be placed on 

 wet blotting-paper at the bottom of a jar, whose mouth 

 must of course be left open. 



III. RESPIRATION. 



249. Respiration. The organic products of photo- 

 synthesis, and of the further metabolic processes starting 

 from the photosynthate, in autotrophic plants sooner or 

 later disappear owing to (1) absorption by animals or 

 parasitic plants, or (2) processes of decay, or (3) respira- 

 tion. We have studied the loss in dry weight undergone 

 by seedlings grown in darkness, and it could easily be 

 proved by experiments that a similar loss occurs in all 

 living plants, whether they are actually growing or merely 

 maintaining life. Since the disappearing material cannot 

 be destroyed, it must escape from the plant in the form of 

 gas. Our experiments on seedlings showed that the loss 

 is mainly a loss of carbon, escaping in the form of carbon 

 dioxide, and this giving-off of carbon dioxide is accom- 

 panied by absorption of oxygen. 



250. Respiroscopes and Respirometers. For the 

 simple demonstration of respiration, it is only necessary 

 to test the gas in a vessel in which active tissues have been 

 enclosed for some time and compare its composition with 

 that of the atmospheric air originally present. A respiro- 

 scope consists of any gas-tight chamber connected with a 

 tube in which the identity of the gas may be ascertained 

 by a visible test with a gas-absorbing reagent. A respiro- 

 meter is simply a similar apparatus adapted by graduation 

 for the quantitative analysis of the gas in the chamber. 



