36 THE SYNTHESIS OF CARBOHYDRATES 



amount of radiant energy absorbed. This, in brief, is Engle- 

 mann's * theory of chromatic adaptation, a theory which 

 has been extended to include many plants, Phaeophyceae, 

 Bacilliarialcs, Bacteria, and terrestrial plants which possess 

 these extra pigments. Gaidukov f in his experiments with 

 Oscillana found that the cultures changed their colour, violet 

 in dim light, to the complementary colour to that in which 

 they were growing ; thus if grown in red light, the new cells 

 would be green, and so on. Whether this is a true example 

 of chromatic adaptation is, perhaps, doubtful, for other 

 authors have found like colour changes to be effected by the 

 conditions of nutrition, a deficiency of iron, by wounding, and 

 so on. 



Richter % considers that the determining factor in the 

 colour of red algae is the intensity rather than the quality 

 of the light, the water soluble pigments playing no part in 

 the photosynthetic process. Wurmser,§ on the other hand, 

 whilst agreeing that the presence of phycoerythrin is an adap- 

 tation to weak light, concludes that it also is an optical sen- 

 sitizer which aids the chloroplasts to absorb green light. 



Harder || studied Phormidium foveolarum, which is purple 

 in blue light, owing to the development of phycoerythrin, and 

 green in red light, owing to the development of phycocyanin. 

 He found that the purple plants have a higher rate of photo- 

 synthesis in blue light than in red, whilst the green forms show 

 a higher rate in red than in blue light. He thus supports 

 Englemann's hypothesis, but his experiments also show that 

 light intensity is of great importance, but even so there is 

 always a higher rate of photosynthesis in the plants which 

 have a complementary pigment. On an earlier page mention 

 has been made of Harder's observation that Phormidium 

 when cultivated in a strong light shows a higher rate of carbon 



* Englemann : " Bot. Ztg.," 1883, 41, 1, 17 ; 1884, 42, 81, 97. 



t Gaidukov : " Preuss. Akad. Wiss.," 1902, 5, 1 ; " Ber. deut. bot. 

 Ges.," 1903, 21, 484, 517, 535. 



J Richter : " Ber. deut. bot. Ges.," 1912, 30, 280. 



§ Wurmser : " Recherches sur l'assimilation cblorophyllienne," Paris, 

 1921. 



|| Harder : " Ber. deut. bot. Ges.,*' 1922, 40, 26 ; " Zeit. Bot.," 1923, 



•5» 305- 



