310 



THE FOOD OF PLANTS 



formed, and that it is certainly not the primary product of carbon dioxide assimi- 

 lation, as to the details of which we are still completely in the dark (cf. Sect. 54). 



Methods. It can easily be shown that bubbles of gas rich in oxygen escape 

 from the cut stems of illuminated aquatic plants, such as Elodea, Ceratophyllum, 

 Potamogeton (Sect. 32), but the leaves of terrestrial plants are less suited for this 

 purpose (Sect. 57). Either the apparatus shown in Fig. 25 may be used, or the 

 gases may be collected by means of the simple arrangement shown in Fig. 41, 

 where the bubbles collect in the filter funnel /, and may be shown to be rich in 

 oxygen by direct analysis or by opening the tap A, and holding a glowing match in 

 the stream of issuing gas. It is easy to show that the rate at which the bubbles 

 are produced is directly dependent upon the intensity of the illumination, and 



FIG. 41. 



FIG. 42. 



Kohl, by counting the number of bubbles evolved and measuring the diameter 

 of each, has been able to apply this method to determine the relative effects of 

 the various rays of the spectrum (Sect. 60) '. By projecting the apparatus upon 

 a screen the formation of the bubbles may be demonstrated to a large audience, and 

 it may be shown that they stop directly lime-water is added (Sect. 32). Masses of 

 green algae may also be used, for these slowly sink in darkness, but in the light 

 they are raised to the surface by the adhering bubbles of evolved oxygen. 



Since the time of Ingenhousz, analyses of the surrounding air have been made 

 in order to determine the gaseous changes incident to the assimilation of carbon 

 dioxide, and a simple apparatus, well adapted for a variety of experiments of this 

 kind, is shown in Fig. 42. A leaf attached to a platinum wire is introduced into 



1 Kohl, Ber. d. Bot. Ges., 1897, p. in. The bubble-counting method was first employed by 

 Dutrochet (Me"moires, &c., Bruxelles, 1837, p. 182) and Sachs (Bot. Zeitung, 1864, p. 363). 



