36o ECOLOGICAL FACTORS, ETC. 



example, it has recently been shown that, under persistently 

 changed conditions a fresh-water 'sun' algae can be converted 

 into a 'shade' area. 



As a result of an exhaustive study of colour in relation to assimi- 

 lation, Montfort (1934) concluded that the quality and intensity of 

 the light form the limiting factors in determining the depth at 

 which an alga can live. These conclusions may be summarized as 

 follows : 



(i) An alga may go deeper in the water the nearer its assimilation 

 curve approaches that of the shade type and the lower is its com- 

 pensation point. (Compensation point = that strength of light in 

 which the minimum assimilation will compensate for respiration.) 

 The better the protoplasmic adaptation to the strong, deep-going, 

 blue-green light waves, the greater is the power of colonizing the 

 deeper areas. Under these conditions, for example, a green shade 

 alga would be able to go to a low^r limit than a red sun alga. 



(2) An alga will go deeper the more its colour is complementary 

 to the spectral composition of the light. Chromatic adaptation by 

 means of Phycoerythrin and Phycocyanin may enable a red alga to 

 have a greater energy absorption in blue-green light than a green 

 alga, even if under conditions of white light the green alga has a 

 greater light absorption than the red. 



GEOGRAPHICAL DISTRIBUTION 



Many of the studies of algal distribution are based on a consider- 

 ation of continuous or discontinuous distribution which are, for 

 convenience, discussed as though they were separate phenomena, 

 although it is clear that no distribution can be absolutely continuous. 

 When, however, it is found that an area in which the localities are 

 fairly close together is separated by the width of a continent or of 

 an ocean from another similar area, then we may talk of discon- 

 tinuous distribution. The problem is rendered more difficult by the 

 unreliability of earlier records and the somewhat scanty literature, 

 especially for tropical and sub-tropical areas. The few studies 

 (Svedelius, 1924; Borgesen, 1934), that have been pubHshed have 

 established certain general features which are briefly summarized 

 below : 



(i) There is a general resemblance between the algal floras of 

 the West Indies and the Indo-Pacific. Vicarious pairs of species 



