792 ZONES AND REGIONS [Pt. Ill, Sect. V 



That there is an increase of vegetation deeper down appears probable, 



considering the sensitiveness to light of most Algae and the conditions in 



the Mediterranean. The luxuriant growth of Algae in the shade of the 

 mangroves also favours this opinion. 



iii. THE BENTHOS OF WARM TEMPERATE SEAS. 



The warm temperate seas have been examined in greater detail than 

 tropical seas as regards the geographical distribution of Algae and have 

 shown the existence of several more or less sharply defined districts. Thus 

 the algal flora of the Red Sea is very different from that of the Mediter- 

 ranean ; and the Australian sea, as regards its flora, is just as peculiar 

 as the Australian land. The differences in the majority of cases appear 

 to depend more on geological evolutionary causes than on existing physio- 

 logical causes. Thus, for instance, we cannot discern what physiologically 

 effective factors should cause the great differences in marine flora on the 

 two sides of the Isthmus of Suez. In other cases, however, it is not incon- 

 ceivable that existing causes, such as salinity, influence on temperature 

 due to currents, and the like, should participate more or less in determining 

 their differences. But the matter has not yet been investigated. 



Of the oecology and distribution of the algal vegetation in the Bay 

 of Naples, Berthold has given a detailed account which appears to be 

 satisfactory in every respect, and which may perhaps be largely applicable 

 to other warm temperate seas. 



The coast of the Bay of Naples is chiefly rocky ; it also exhibits muddy 

 and sandy stations. 



The emerging belt, as everywhere else, is found only on a stony sub- 

 stratum, and growing luxuriantly in it are many species that are absent or 

 rare in the submerged belt, for instance Rhodophyceae belonging to the 

 genera Porphyra, Ceramium, Callithamnion, Bangia, also various Chloro- 

 phyceae, especially species of Ulva. 



The submerged belt possesses a much greater breadth and a much richer 

 flora than does the emerging belt ; near Capri, its vegetation is still 

 luxuriant at a depth of 120-130 meters. The sandy soil is covered by 

 Posidonia oceanica forming meadows, which are still continuous at a depth 

 of 60 meters, but which, from 80-100 meters, show only isolated plants. 

 Down to a depth of 15 meters, Caulerpa prolifera (Fig. 481) accompanies 

 Posidonia. Yet the Caulerpa shows a preference for mud where Zostera 

 marina and Z. minor, here and there, grow in dense crowds. Other 

 Algae in these formations are epiphytes on sea-grasses, or lithophytes fixed 

 to individual stones or shells. Even a moderate movement in the water 

 disturbs the loose soil and excludes all vegetation from it. 



The lithophyte-vegetation of the submerged belt is rich in forms, and 

 is differentiated into manifold formations, the distinctions between which 



