PLANTS WITHOUT CARBON DIOXIDE 231 



and will throw off an equal amount of oxygen, and the experiment 

 may be carried out without renewing the air. In order to give 

 more normal conditions, however, the pump is started so that a 

 slowly moving stream of air is drawn through the material contain- 

 ing potassium hydrate, which removes all of the carbon dioxide and 

 about all of the water at the same time. The evaporation from 

 the sponge renews the moisture and makes an atmosphere of aver- 

 age humidity. At the same time the solution of potassium kept 

 in the bell-jar absorbs the carbon dioxide given by the plant as a 

 result of respiration. The ventilation of the bell-jar need not be 

 carried on continuously. The pump may be allowed to run an 

 hour or two in the morning, and the same period in the afternoon. 

 It will be best to interpose a wash bottle containing a solution of 

 potassium hydrate between the pump and the bell-jar to prevent 

 any possible contamination by backward movement of unfiltered 

 air. Replace this wash-bottle by one containing barium or calcium 

 hydrate occasionally during the ventilation and note whether the 

 liquid becomes milky because of the passage of the air from the 

 bell-jar. If it does, the presence of carbon dioxide in the jar is 

 proven and the experiment must be amended to exclude it by 

 a more thorough filtering processes. 



Observe the behavior of seedlings of Phaseolus, or Zea, under 

 the conditions described. A certain amount of growth is carried 

 on by the use of food stored in the seed but when the plant be- 

 comes dependent upon photosynthetic activity it perishes. Com- 

 pare the behavior of seedlings with that of small bulbous plants. 

 The shoots of large plants may be allowed to grow through a 

 perforated glass plate and then covered with the bell-jar. The 

 opening around the plant may be closed with a cork sealed with 

 wax to the glass, and made tight around the plant by a seal of mer- 

 cury and water (See Fig. 113). Ten to fifteen days will be neces- 

 sary to reach conclusive results with most plants. Make a micro- 

 scopical examination of the structure of the leaves after the effects 

 of the lack of carbon dioxide have become visible, and compare 

 with that of normal leaves. 



