ECOLOGICAL FACTORS 357 



and the^H of most pools, except perhaps the highest, will rarely be 

 outside these values. Table XXII shows that the intertidal algae, 

 which form the principal component of the tide pool vegetation, 

 exhibit a greater range of osmotic tolerance than those from deep 

 waters, although season and time of day is important in this respect 

 because either may bring about changes in the concentration. 



It would appear that in spite of all this careful work we are still 

 far from understanding the factors that control the vegetation of 

 tide pools because many of the algae will tolerate the range of 

 conditions which are likely to be found in such places. The algae of 

 the tide pools can be placed into one of the four following groups : 



(i) Those which are sublittoral and which also occur in the tide 

 pools. 



(2) Those which grow near the ebb line and reach their upper 

 limit in the pools. 



(3) Those which grow in both the intertidal zone and the pools. 



(4) Those confined wholly to the rock pools. 



Klugh and Martin (1927) studied the growth rate of various 

 algae in relation to submergence by measuring plants and then 

 tying them to floats which were suspended in the water at different 

 depths. After some months the floats were pulled up and the plants 

 were remeasured. Light, temperature and salinity all vary with 

 depth, but the last two factors vary so little that it is doubtful 

 whether they can be of any significance. Light, however, is very 

 rapidly absorbed by the water, so that at about 2 m. down only 

 25 % of the surface light has penetrated. The curves (fig. 205) show 

 that maximum growth occurred between i and 2 m., and it would 

 seem that whilst the light was perhaps too bright at the surface, 

 nevertheless it soon became limiting at depths which varied for the 

 different species. On the basis of a rather limited number of species 

 and experiments it was concluded that the Rhodophyceae are no 

 better adapted to greater depths than the Chlorophyceae and 

 Phaeophyceae, but in the light of more recent experiments it would 

 seem that this conclusion must be revised. Using coloured Hght 

 under experimental conditions Montfort (1934) showed that there 

 was an essential confirmation of Englemann's complementary 

 theory, which states that the colour of an alga is complementary to 

 the colour of light that it absorbs (cf. also p. 293). The phycoery- 



