CHAP. XLI LITHOPHILOUS BENTHOS 171 



supply of oxygen and food-material influence the distribution of associations. 

 The algal vegetation on exposed coasts as a rule differs considerably from 

 that found on sheltered coasts. In this connexion reference should be made 

 to Hansteen's l investigations of the flora outside and inside the Norwegian 

 rocky shoals and Borgesen's 2 on the Faroe Isles. 



Hedwig Loven 3 investigated the respiration and the gas contained in 

 the vesicles of algae, and came to the following conclusions : 



The gas inside fucaceous vesicles is different in composition from that 

 of the air in the water ; 



The amount of oxygen is at the maximum at midday, and at a minimum 

 during night-time ; 



Algae can extract from water every trace of oxygen, but they live for a 

 tolerably long time in water devoid of oxygen, and excrete into it a con- 

 siderable quantity of carbon dioxide ; if oxygen be lacking in the water 

 then they can completely exhaust the oxygen contained in the vesicles. 



Light. In the first place intensity of light is of significance ; the Green 

 Algae are those most photophilous, and, according to Kjellman, it may 

 be for this reason that they are enfeebled as well as scanty in the north 

 Arctic Ocean (though they are luxuriantly developed along rocky coasts 

 of Greenland.) The deeper one goes the more light is absorbed, and the 

 fewer become the species until at last they are absent. According to Berthold, 

 Florideae are generally shade-loving plants. Berthold 4 found at Naples a 

 luxuriant algal vegetation at a depth of 120-130 metres, whereas in the Arctic 

 and North Atlantic Oceans only a poor vegetation subsists at a depth of even 

 50-60 metres. 5 The differences in their demands for light cause algae to 

 be distributed in zones according to depth. 6 



Then the colour of light changes with the depth, 7 and correlated with 

 this change are the colour of the algae and their distribution in ' regions '. 

 Lyngbye in 1836 established the ' regions ' of the Ulvaceae, Florideae, 

 and Laminariaceae ; Agardh in 1836 and Orsted in 1844, established 

 those of the Green, Brown, and Red Algae. But Orsted in 1844 was the 

 first to assume the connexion between colour of light and depth of the strata 

 in which algae occur ; in the Ore-Sund he recognized the following ' regions ' 

 commencing at the surface and descending : 



1. Regio algarum viridium S. Chlorospermearum. 



Sub-regio Oscillatorinearum. 

 Sub-regio Ulvaceamm. 



2. Regio algarum olivacearum S. Melanospermearum. 



Sub-regio Fucoidearum et Zosterae marinae. 

 Sub-regio Laminariarum. 



3. Regio algarum purpurearum S. Rhodospermearum. 



Kjellman has divided the algal vegetation off the Swedish part of 

 the Murman coast, and in other seas, into three ' regions ', which run 

 parallel with the coast, and are in turn composed of a great number of 

 small * formations ' (i.e. associations), according as one or another species 

 predominates. The three regions are the following : 



1 Hansteen, 1892. 2 Borgesen, 1905. 3 Loven, 1891. ' 



4 Berthold, 1882; see Oltmanns, 1905. 6 Rosenvinge, 1898; Borgesen, 1905. 

 K Concerning algal vegetation in caves of the Faroe Isles, see Borgesen, loc. cit. 

 7 See p. 150. According to Gaidukow, 1904, the colours of algae are to be 

 regarded as adaptations to the quality of the light present. 



