CHAP. XLi LITHOPHILOUS BENTHOS 173 



the commencement of summer, may in arctic seas persist throughout 

 summer.^ In Danish waters the marine algal vegetation in summer 

 differs widely from that in winter,- and even in the more southern latitudes 

 of Naples the same has been noticed.^ In the latter place illumination 

 and breakers are the determinants, but in higher latitudes temperature 

 certainly plays a more important part. 



A pecuhar group, that of the Diatomaceae, merits special notice, as 

 it includes forms deviating from all other plants. Among them are 

 the ground-diatoms showing various growth-forms, including motile 

 forms which creep over the substratum (stones or other algae), and 

 stalked non-motile forms, which especially inhabit the marginal zones 

 of salt-water, are easily detached and then can intermingle with plankton."* 

 These communities of diatoms may perhaps be regarded as constituting 

 a separate formation which includes many associations. 



Moist rocks on sea and land may bear a vegetation which is transitional 

 between submerged rock-vegetation and land- vegetation, and may give 

 rise to a special formation. Inland nereid-communities are dependent 

 upon great atmospheric humidity and trickhng water, and they therefore 

 develop luxuriantly only near waterfalls, whose spray habitually wets 

 the rocks, and in countries where atmospheric precipitations are heavy 

 and fall throughout the year (as in Java), and in the cloud-belt of moun- 

 tains. On rocks that are wet by fresh water there may be formed a 

 spongy felted carpet of algae (including Trentepohha, Rhodochorton 

 islandicum, R. purpureum and others), mosses, ferns, and other herbs ; 

 and there may even be found small shrubs that are always wet or dripping 

 with water. On rocky coasts foam of the breakers may be carried 

 especially high, and in these same places marine algae, such as species of 

 Prasiola, Ulothrix and Enteromorpha, Calothrix scopulorum, also Bangia 

 fusco-purpurea and Hildenbrandia rosea, may occur far above the high- 

 tide mark.^ Thus arises a kind of supra-littoral ' region '. Various crus- 

 taceous lichens, including Verrucaria maura, are intermingled therein. 

 The oecology of this community differs from that in water in so far as 

 the constituent species must be fitted to endure greater dryness than in 

 the case of submerged types. 



CHAPTER XLII. BENTHOS OF LOOSE SOIL 



The structure of soil has already been described,^ but in the soil 

 now under discussion the interstices arc filled with water, and air occurs 

 in extremely small quantities or not at all. 



The texture of the soil may vary, being mud, clay, or pure sand, which 

 is mostly quartz-sand or, in the tropics, coral-sand, and may be mixed 

 with marine shells and stones, more or less small according to the violence 

 of the waves. These differences in texture cause floristic distinctions, 

 though hardly anatomical or morphological ones. But nothing furtlier 

 is known in regard to this matter. An exceptional kind of soil is mud 

 that consists of dead organic matter. 



' Rosenvinge, 1898. 



' Kjellman, Rosenvinge, and others; as cited by Oltmanns. 1905. 



' Berthold, see Oltmanns, IQ05. * Schiitt, sec Oltmanns, 1905 ; compare p. i 56. 



' Rosenvinge, 1903, see Oltmanns, 1905; Borgcsen, 1905. ' See Chap. X. 



