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. 



