214 BOTANY pakt i 



sunlight. If we assume that the carbonic acid gas of the atmosphere 

 (carbon dioxide, CO.,) becomes on its sohition in the cell H.^CO^, the 

 formation of sugar would take place in two stages. In the first, ox3^gen 

 would be given off and formaldehyde formed, 



H2CO3-H2CO + O2 



In the second stage the aldehyde is polymerised to sugar, 



6H,C0 = C«H,,06 



If we start from carbon dioxide we must assume that water enters into 

 the reaction according to the formula 



eCOo + GH.O = CJl^Pe + 6O2 



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. The oxygen given off can, however, even when it is only de- 

 tected qualitatively, be used as an indicator of the decomposition of 

 the carbonic acid. Thus, when a plant is enclosed along with phos- 

 phorus 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 drawing conclusions as to the production 

 of oxygen by a green plant is afforded by the movements of certain 

 Bacteria Avhich previously lay motionless on the Avater. The 

 clearest demonstration of assimilation is obtained by using certain 

 Avater-plants such as Elodea or Potambgeton. 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. 194), 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, 

 causing there an increase of the pressure, and this is the cause of the appearance of 

 bubbles of gas at every wounded surface. 



It was by the excretion of bubbles of oxygen that Ingenhouss 

 was first led to recognise the nutritive functions of leaves. Ingenhouss 

 himself and further Priestley and Th. de Saussure had established 

 the main features of the decomposition of CO2 in the green plant by 

 the end of the eighteenth and beginning of the nineteenth centuries i^'^). 

 This discovery is one of the most important in the progress of the 

 natural sciences. It was by no means easy to prove that the invisible 



