PHYSIOLOGY 



295 



pigments only help in the amount of light absorbed and not in its 

 utilization. We have already seen earlier that other workers, who 

 have studied members of the individual groups, have arrived at a 

 similar conclusion since they regard the coloured pigments as 

 acting in the same way as a colour screen. Apart from the physical 

 adaptation, in the sense of complementary Hght absorption, there is 



700 600 



700 



600 500/^/i 400 

 B 



50Qjuju,A00 



A 



Fig. 188. Absorption curves of A, Monostroma, B, Delesseria, at different 

 depths. (After Seybold.) 



also a physiological adaptation to strong and weak light and to long 

 and short wave light. The algae can be placed into two groups 

 depending upon their responses to strong and weak light, or to 

 long wave and short wave light. This problem, however, is followed 

 up more closely in the later chapter on algal ecology (cf. p. 359). 



^SYMBIOSIS 



The most striking and well-known examples of symbiosis in the 

 algae are provided in those cases where the plants are associated 

 with animals, especially Coelenterates, or with fungal threads, as in 

 the common lichens. Apart from these examples, however, there 

 are other cases which are not so well known, largely because they 

 are not so common or so conspicuous. Gloeochaete, for example, is 

 a colourless genus of the Tetrasporaceae which possesses blue 

 green bodies that look like chromatophores, though they are really 

 a symbiotic blue-green alga. Gleucocystis is a colourless genus of 

 the Chlorococcales in which a symbiotic member of the Cyano- 

 phyceae also forms blue green "chromatophores" that appear as a 

 number of curved bands grouped in a radiating manner around the 

 nucleus. In this case the illusion is further enhanced because they 

 break up into short rods at cell division. It has so far proved 



