SYNTHESIS, STORAGE AND BREAKDOWN 121 



oxygen, produced by photosynthesis. Some of the oxygen so produced 

 diffuses away from the plant in solution, but owing to the low solubility 

 of the gas in water it accumulates under pressure in the gaseous form 

 inside the intercellular spaces and escapes from any cut surface. The 

 rate of production of bubbles gives a rough index of the rate of photo- 

 synthesis, and experiment would show that the bubbling slows down 

 if the light-intensity be considerably reduced, and would stop if 

 water free of carbon dioxide were to be supplied to the plant. Or 

 again the method could be employed to compare the rates of photo- 

 synthesis in light of different colours, and to demonstrate the fact 

 that up to a point increase in temperature hastens photosynthesis, 

 if other factors are favourable. Another method, chiefly perhaps 

 of historical interest, for demonstrating the liberation of oxygen 

 involves the use of certain bacteria which show movements only if 

 supplied with oxygen. If a filament of Spirogyra (a simple water- 

 plant, see pp. 372-3) is mounted in water containing the bacteria, and 

 air excluded, the movement of the bacteria continues in the vicinity 

 of the plant, provided it be exposed to light. The production of oxygen 

 is thus indicated. A special tendency has been detected for the bac- 

 teria to accumulate near the chloroplasts. Other experiments 

 involving the use of methods of gas analysis show that the volume of 

 oxygen produced in a given time by photosynthesis is equal to that of 

 the carbon dioxide absorbed. 



It is impossible to attempt here any detailed consideration of the 

 chemical changes that may be involved in the building-up of sugars 

 from carbon dioxide and water. There is no doubt that the synthesis 

 involves a number of intermediate reactions, and though actually we 

 have little information about these, it has often been supposed that 

 first the substance formaldehyde is produced according to the 

 equation : QQ ^ + Ha0 = h • CHO + 2 



and that six molecules of the aldehyde then become united together 

 to give sugar : 6H • CHO =C 6 H 12 6 - 



This latter reaction is easily effected in the test tube by subjecting 

 formaldehyde to the action of alkalies. Each of these steps may 

 itself involve a series of intermediate reactions. The evidence in 

 favour of the formaldehyde theory is rather slender, and indirect rather 

 than direct in nature ; but it should be realised that the investigation 

 of the chemistry of photosynthesis is beset with great difficulties. 

 There are two obvious lines of attack, namely, to test for the presence 



