114 THE GREEN PLANT CELL 



of oxygen diffusing into the water, is then introduced 

 in the neighbourhood of the illuminated algal thread. 

 The minute cells of the bacteria swarm towards and 

 cluster round those portions of the thread illuminated 

 by the coloured rays corresponding with the chief 

 absorption band of chlorophyll, especially in the red 

 (Fig. 9). 



This behaviour of the bacteria shows that free oxygen 

 (one of the products of the process of photosynthesis) 

 is being given off most actively at those points ; and 

 consequently that those particular rays are the ones 

 most active in photosynthesis. 



Sugar, probably glucose, is the first product of the 

 process of photosynthesis that can be definitely recog- 

 nised in, the green cells of the leaf, free oxygen being 

 liberated at the same time. These substances are only 

 formed in the presence of a certain intensity of the proper 

 rays of the spectrum (pure white sunlight contains all 

 the rays), and in the presence of carbon dioxide and 

 water. We may represent the general equation of 

 photosynthesis thus : — 



6CO2 + 6H2O = CeHi^Ofi + 6O2 



It is likely, however, that formaldehyde is first formed : — 



CO2 + H2O = CH2O + O2 



and that the formaldehyde is then condensed to form 

 glucose : — 



6CH2O = CeHiPe 



but this is uncertain, since formaldehyde has not been 

 demonstrated in the leaf. Nevertheless the " reduc- 

 tion " of CO2 inpresence of water to form formaldehyde 

 and also the condensation of formaldehyde to form 

 glucose are both operations that can be performed in 



