278 PHYSICAL FACTORS [ch. 



being we may safely say that it has not been satisfactorily 

 determined in any one case whether its development is either 

 an advantage or a disadvantage to the plant." It is therefore 

 clear that the attractive theory that red coloration is developed 

 by water plants as an adaptation to their mode of life, must be 

 definitely abandoned, unless further evidence for its validity 

 can be produced. 



Although water plants live, on the whole, in a more equable 

 and temperate climate than land plants, yet they are liable in 

 winter to one very severe ordeal the freezing of the water in 

 which they occur. Some escape this trial by their habit of 

 sinking to the bottom in the cold season, while others are able to 

 withstand a temperature below freezing point for a long period, 

 especially when they are in the turion or seed phase^. 



The illumination, to which submerged plants are exposed, 

 is as much affected by the medium as are the thermal con- 

 ditions. Free-swimming water plants and those with floating 

 or aerial leaves, on the other hand, receive light in much the 

 same way as land plants; as a result of their situation, the leaves 

 are often exposed to all the available sunshine, mitigated by no 

 shade whatever. Such plants thus present no problems of 

 special interest in connexion with their light conditions, and 

 they may be disregarded in the present discussion, which will 

 be confined to those that are more or less completely sub- 

 merged. 



The light which reaches a submerged shoot has been reduced 

 by four factors reflexion from the water surface, absorption 

 by the water, and darkening due to certain substances in solu- 

 tion or to solid particles in suspension^. The absorption and 

 darkening may be very considerable in the less limpid waters. 

 It has been shown by experiments with a recording galvano- 

 meter that 60 per cent, of the light may be absorbed by the 

 first two metres^. Some observations made in the Lake of 

 Geneva"*, with regard to the limit of visibility, show that a 



1 See pp. 220, 243. 2 Goebel, K. (i 891 -1893). 



3 Regnard, P. (1891). * Forel, F. A. (i 892-1904), 



