THE ALGAE 185 



comes the giant Saccorrhiza bulbosa, growing in several fathoms of water and 

 producing thaUi more than 12 ft. long. 



Such plants when growing in large numbers cast a gloom through the 

 water, and among the fronds there is only sufficient light for members of the 

 Rhodophyceae to live. In the forests formed by these Laminarias every rock 

 and cranny is covered by small Red Algae. Only these Algae can live and 

 thrive under such conditions, by virtue of the red pigments which they 

 possess, which are supposed to enable them to absorb the predominating 

 bluish light under water. We need not enumerate here the various species 

 which occur, but we must realize that under these conditions they are much 

 more luxuriant than the stunted specimens we find sheltering under Fucus 

 plants on the higher zones of the shore. 



It has been observed that certain species of seaweeds show a tendency 

 to move downwards towards lower zones of the shore during the winter 

 months, while others, in contrast, migrate landw^ards. 



Quite apart from the larger Algae, whose distribution we may study on 

 the seashore or the lakeside, there are countless microscopic species, which 

 contribute to that floating population of organisms collectively spoken of as 

 plankton. Many of these species multiply at a surprising rate, sometimes 

 covering the whole surface of a pond in a few days with an almost pure culture 

 of some particular organism, only to disappear again as quickly as they 

 have come. 



Many and complex are the problems associated with the distribution of 

 the Algae, but sufficient has been said to indicate the scope of the subject 

 and, it is hoped, to stimulate the reader to further observations of his own. 



The Metabolism of Seaweeds 



The habitat of a seaweed differs greatly from that of the majority of land 

 plants so that it is only natural to find that they have a number of physiological 

 peculiarities. 



When submerged they receive light that has been filtered through varying 

 depths of sea water and which is, in consequence, deprived of most of the red 

 and yellow rays which are those chiefly absorbed by chlorophyll. The view 

 has been widely held that the red and brown pigments are functionally 

 important in that they can absorb the blue and green rays of sunlight and so 

 obtain photosynthetic energy for the plant when under water. Red Algae 

 can grow down to depths of about 1 2 fathoms on our coasts and much deeper 

 in clear seas, but in dull weather the maximum rate of photosynthesis appears 

 to be at or near the surface and in bright weather about 2 fathoms down. 

 This does not altogether bear out the adaptation idea, and suggests that the 

 red colour is possibly only a protection for the chlorophyll against over- 

 bright sunlight, and that the seaweeds are simply shade plants with particularly 

 sensitive chlorophyll. It is perhaps worth noting here that only chlorophyll 

 a occurs in the Red and Brown Algae, the other form, chlorophyll ^, 

 being absent. On the shore the dominant physiological factor is undoubtedly 

 exposure, which includes both bright light and desiccation. The rate of drying 



7A 



