THE LEAF 



129 



carbonic acid by the plant can be demonstrated any 



time to a large audience by means of an improved 



magic lantern much in vogue nowadays — which throws 



on the screen a 



magnified image 



of the plant and 



the tube in which 



the gas given off 



by the plant is 



investigated. 



Here is one of the 

 most convenient 

 forms of this ex- 

 periment (fig. 38) . 

 A small glass 

 cell made of a 

 glass tube bent 

 into the shape of 

 a horse-shoe (e) 

 and two glass 

 plates (d) forms 

 a kind of aqua- 

 rium in which 

 common water 

 plants are grown. 

 If we arrange for a 

 sufficiently strong 

 source of light, 

 either sunlight, 

 electric, or lime- 

 light, we can 

 throw upon the 

 screen an image 



seven or more feet across of this minute aquarium 

 (diminished by one half in fig. 38), and we observe 

 at all places where the stems or the petioles of the 

 leaves are cut across that a curious phenomenon is 



I 



Fig. 38. 



