11.] RESPIRATION AND ASSIMILATION 31 



bottle is no longer capable of supporting combustion. 

 Now stopper-up the bottle again and stand it in the 

 light for two days ; again test the air it contains with a 

 taper, and it will be found to support combustion. In 

 the light the green leaves of the barley have split up 

 the carbon dioxide and replaced it by oxygen, until the 

 air will once more support combustion. The bottle may 

 be replaced in the dark until respiration has again 

 loaded the air with carbon dioxide, whereupon it may 

 again be brought into the light and the air reoxygenated 

 by the assimilation carried out by the green leaves ; 

 these alterations may be repeated several times, until 

 at last the barley begins to die for lack of food. Under 

 slightly more natural conditions, however, such a cycle 

 of respiration and assimilation can be continued for 

 very long periods. For example, in the Rothamsted 

 laboratory a sample of soil weighing several pounds was 

 put away in a rather moist condition in 1874 i^ 3. gallon 

 bottle, which it three parts filled. The bottle was corked, 

 the cork waxed and covered with a lead capsule. A few 

 years afterwards a fern, Aspleniuin nigrum^ was noticed 

 to be growing within the bottle, having developed from 

 a spore which must have been present in the undried 

 soil ; this fern has continued to grow, and at the present 

 time (19 10) it occupies nearly the whole of the vacant 

 upper part of the bottle. No air can either enter or 

 leave the bottle ; at first some doubts were entertained 

 as to the impermeability of the sealed cork, but the 

 whole of the cork and the neck of the bottle have been 

 enclosed in a block of paraffin wax, which was melted 

 and cast on to its present position. The bottle stands 

 on a low shelf in a building lighted from the roof, the 

 illumination it receives is only good for a few hours on 

 summer days, when a little direct sunshine reaches it. 

 This illumination in the summer is, however, enough to 



