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



Berthold at Naples, and occur also elsewhere, for instance in the Riviera. 

 The species that possess in a high degree the faculty of adapting themselves 

 to the intensity of light temporarily prevailing can endure without injury 

 unequal degrees of brightness, whereas certain Florideae (Palmophyllum, 

 Cruoriopsis, and others), owing to defective plasticity, are so shy of light 

 that they are restricted to the shade even at a depth of 60 meters. 



The very pronounced periodicity of marine vegetation in the Mediter- 

 ranean, and probably in other warm temperate seas, is likewise essentially 

 associated with the light. 



In accordance with tJic conditions of illumination prevailing, it is a 

 universal rule, in the Mediterranean Sea, that the vegetative season largely 

 coincides, near the surface, with the winter and spring mouths, but in deeper 

 water with the autumn and summer mouths. 



In particular, the change is sharply revealed in connexion with the 

 Phaeophyceae, which at midsummer prevail over the Florideae in deep 

 water, whereas at other seasons the Florideae predominate in all poorly- 

 lighted situations. 



Exposed stations that support a luxuriant vegetation during winter 

 become barren in summer. Many forms of Algae belonging to the upper 

 belts are sun-plants in winter ; for instance, species of Plocamium, Calli- 

 thamnion, Phyllophora nervosa, P. Heredia, Cutleria. Many species appear 

 in shallow water in winter, but in deep water during summer ; for instance, 

 Stilophorae, Nereia filiformis. Especially worthy of note are those species 

 which, in accordance with the prevailing differences in the illumination, 

 each exhibit a winter-form and a summer-form differing as regards their 

 hairs and their method of branching; for instance, Stypocaulon scoparium, 

 Halopteris filicina, and others. 



Other factors besides light take either a quite subordinate part in the 

 periodic phenomena or none at all. This is the case with temperature, 

 which has no appreciable effects, whereas changes in movement due to 

 the seasons are of greater significance at certain spots near the surface. 

 Thus the breakers on freely exposed rock in the outer Bay of Naples are 

 weaker during summer than during winter and spring. Consequently 

 the rocks more exposed to surf during late spring and early autumn, 

 and even in places during summer, bear a richer vegetation, despite 

 the more intense light, than they bear during winter. 



The preceding remarks, essentially based upon Berthold's work, refer 

 exclusively to the photic region. As regards the dysphotic region of the 

 Bay of Naples, we are less well informed, because Berthold did not devote 

 attention to the Diatomaceae and other microphytes that chiefly occur 

 there. He merely states that below a certain depth, varying according 

 to the illumination of the water, aspect, movement, and similar causes, 

 algal life is poor. Nothing whatever is known regarding the aphotic 



