172 HYDROPHYTES sect, iv 



1. Littoral ' region ' ; 



Stretches between the high-tide and the low-tide marks, and includes 

 many Green Algae, Brown Algae, and some Red Algae. At low tide these 

 lie uncovered; many may be described as nearly amphibious, for they 

 become dry on bright sunny days. In extreme arctic seas the rubbing 

 action of the masses of ice prevents any strong development of this ' region '. 



2. Sub-littoral ' region \' 



Ranges from below low-tide mark down to a depth of twenty fathoms 

 (40 metres) ; here algae of all colours are represented, but Green Algae 

 cease and Red Algae become more numerous with increased depth. 



3. E littoral ' region ' ; 



Is below the preceding and descends as deep as light ; it is poorer 

 in species and individuals ; moreover, the latter are smaller and distorted, 

 as Lyngbye has already noted. 



Hansteen and Gran on the whole approve of the preceding scheme ; 

 but Reinke, basing his conclusions on investigation of the Baltic Sea, 

 from which ' region 3 ' is absent, suggests a further partition of regions i 

 and 2 into two sub-divisions each. At a depth of 4 metres many species 

 find their lowest limit. Rosenvinge and Borgesen ^ indicate that the 

 littoral ' region ' ' extends far beyond the highest tide-mark, in the Faroes 

 even in some places about 25-30 metres beyond ', and they set the ' lowest 

 limit of the sub-littoral region above the lowest ebb-marks '. Rosenvinge 

 and Borgesen do not recognize any elittoral ' region '. 



Some algal hairs are assimilatory, as in Desmarestia aculeata and 

 Chorda tomentosa, but others are colourless, especially among Red Algae. 

 Those of the latter kind are strongly developed when the light is more 

 intense, and Berthold has made the hardly probable suggestion that 

 their function is to regulate the illumination ; they would seem rather 

 to be respiratory or absorbing organs. ^ 



The factors already enumerated affect the vegetation notably as a 

 whole but also in detail, and, possibly with the co-operation of other 

 factors (kind of rock, and other topographical features), they aid in evoking 

 a number of miniature topographical distinctions and a number of associa- 

 tions, which may be profusely intermingled and may owe their appearance 

 mainly to one or a few species that compose their main mass.^ (Phycolo- 

 gists often apply the term ' formations ' to these miniature purely floristic 

 groups ; but it must be noted that ' formations ' should be oecologically 

 founded upon the forms of the algae, and that true ' formations ' may here 

 be quite out of the question. These groups then are, at least tentatively, 

 to be regarded as associations.) Within extensive communities of large 

 algae for example, among Laminaria stems many epiphytes and many 

 humbler forms find suitable homes ; thus an underlying vegetation of 

 plants requiring less light arises, just as in a forest. 



As the various factors enumerated act with unequal intensity at 

 different seasons of the year, there arises a differentiation in time of the 

 development of the nutritive and reproductive organs. Each species 

 of marine alga seems to have its definite season of development, which 

 may differ with the latitude : species that in Denmark disappear with 



^ Rosenvinge, 1898 ; Borgesen, 1905. 



' See p. 168, and Wille, 1885 ; Rosenvinge, 1903. 



^ Kjellman, 1878 ; Hansteen, 1892 ; Borgesen, 1905 



