226 NUTRITIVE METABOLISM 



amount is measured out for refilling the jars, since some of the 



calcium salts used are only sparingly soluble, and will collect at 



the bottom of the vessel.."- 



tmgs of Salix, Begonia, or any convenient plant may be 



used instead of seedlings. Half of the length of the cutting 



should be immersed in the solution, and it should be cared for 



otherwise, as a seedling. 



Successful water cultures of aquatic plants have been carried 



out with Lemna and Philotria, but solutions of less concentration 

 should be used. A definite number of 

 fronds of Lemna should be placed in an 

 open vessel or small glass aquarium under 

 proper conditions for at least six weeks 

 and then the multiplication of the indivi- 

 duals noted. Shoots of Philotria should 

 be measured and roughly sketched, then 

 cultivated for a similar length of time in 

 an open vessel containing the solutions. 

 A second measurement and sketch will 

 afford a comparison which will determine 

 the effect of the substances tested. 



293. Absorption and Use of Carbon. The 

 air contains about 28 parts carbon diox- 

 ide in 100,000, and all natural waters 



and soils are more or less saturated 

 FIG. in. Aspirator. .... 



with it. As a consequence of this uni- 

 versal distribution, and of the fact that it is constantly liberated 

 in the tissues as the result of respiration, the sap of the plant 

 always contains some of this substance which is slowly diffused 

 outwardly. In cells containing chlorophyl, and perhaps etiolin, 

 however the carbon dioxide is subjected to forces which dis- 

 integrate it and set free a portion of the oxygen which it con- 

 tains. This excretion of oxygen and decomposition of carbon 

 dioxide takes place only when the plasma containing the chloro- 

 phyl is exposed to light. The energy necessaiy to accom- 



