248 



BOTANY 



PART I 



acid gas of the atmosphere (carbon dioxide, C0 2 ) becomes on its solu- 

 tion in the cell H 2 C0 3 , the formation of sugar would take place in two 



stages. In the first, oxygen would 

 be given off and formaldehyde ( 28a ) 

 formed : 



H 2 CO S 



H 2 CO 



2 . 



In the second stage the aldehyde 

 is polymerised to sugar : 



In any case, for every volume 

 of carbon dioxide which disappears 

 an equal volume of oxygen makes 

 its appearance. It has been shown 

 by eudiometric measurements that 

 this is the case (WILLSTAETTER). 

 The oxygen given off can, how- 

 ever, even when it is only detected 

 qualitatively, be used as an indi- 

 cator of the decomposition of the 

 carbonic acid. Thus, when a plant 

 is enclosed along with phosphorus 

 in a space free from oxygen and 

 exposed to light, the formation 

 of oxygen is shown by the white 

 fumes given off from the phos- 

 phorus. Another means of draw- 

 ing conclusions as to the production 

 of oxygen by a green plant is 

 afforded by the movements of 

 certain Bacteria which previously 

 lay motionless in the water (p. 

 331). The clearest demonstration 

 of assimilation is obtained by using 

 certain water plants such as Elodea or Potamogeton. If cut shoots or 

 leaves of these plants are submerged in water and exposed to light, a 

 brisk continuous stream of bubbles comes from the cut surface. If the 

 gas is collected in considerable quantity in a suitable apparatus, e.g. in 

 a test-tube (Fig. 247), it can be shown to consist not of pure oxygen 

 but of a mixture of gases rich in oxygen ; a glowing splinter bursts 

 into flame in the gas. 



The appearance of the bubbles of oxygen is explained in this way. The carbon 

 dioxide dissolved in the water enters the green cells of the plant by diffusion and is 

 there decomposed. The oxygen given off is much less soluble than carbon dioxide 

 and therefore appears in the gaseous form. It passes into the intercellular spaces, 



FIG. 247. Evolution of oxygen from assimilating 

 plants. In the glass cylinder C, filled with 

 water, are placed shoots of Elodea canadensis ; 

 the freshly-cut ends of the shoots are intro- 

 duced into the test-tube R, which is also full 

 of water. The gas bubbles B, rising from the 

 cut surfaces, collect at S. H, stand to sup- 

 port the test-tube. (After NOLL.) 



