36 EXPERIMENTAL PLANT PHYSIOLOGY. 



of aqueous vapor, and .04 part of carbon dioxide. These 

 gases are in different proportions in the plant and conse- 

 quently a constant diffusion through the outer membrane 

 takes place. Some cells containing substances which have a 

 high osmotic equivalent for oxygen absorb it from the air. 

 In like manner cells containing chlorophyll take up carbon 

 dioxide during the daytime. Gases will readily diffuse 

 through a membrane, yet cannot be forced through it by 

 pressure. 



EXPERIMENT 42. 



DIFFUSION OF GAS THROUGH EPIDERMIS. 



Smooth one end of a glass tube with an internal diameter of .5 

 cm. and a length of 30 cm. in a flame. Select a smooth and perfect 

 grape. Take off the skin and clean the pulp from the inside. 

 Place over the end of the tube, bringing the edges down and fasten- 

 ing closely to the tube by a small cord. (Fig. 32.) With sealing- 

 wax secure the edges to the glass in such a manner as to be " air- 

 tight." Test by placing in water and forcing air in at the other end. 

 If no bubbles escape, fill the tube with water and invert in a vessel 

 of mercury. Displace the water with carbon dioxide and note the 

 height of the mercury column daily for a month. By the diffusion 

 of the carbon dioxide through the membrane the column of mercury 

 may be raised as high as 26 cm. 



Remark. In inverting the tube when full of water no air must be 

 allowed to gain entrance. To obtain carbon dioxide use the apparatus de- 

 scribed in Experiment 57. Marble and hydrochloric acid should be used 

 instead of zinc and sulphuric acid, as there described. 



28. Absorption of Hydrogen, Oxygen, and Nitrogen. The 

 absorption of hydrogen, oxygen, and nitrogen, and their 

 synthesis into food are so closely connected with other 

 complex metabolic processes that a consideration of the 

 separate action in each case is somewhat difficult. The 

 manner in which carbon is obtained and used is, however, 

 a fairly distinct process. 



