Chap. II] 



MARINE VEGETATION 



799 



Change in the fronds of the Laminariae is also associated with the season ; yet 

 these plants are never completely devoid of fronds (Fig. 486). 'At the end of 

 October the formation of sori begins in individual Laminariae, and at the end of 

 December, if this has already become general, the first steps towards a change in 

 the frond are apparent. Between the stem and the base of the frond there is inter- 

 calated the new frond in the form of a small (flattened) convex expansion, which, 

 gradually growing as the stem elongates, finally attains a considerable size (up to 

 four meters in L. saccharina) in March or April. During this process the formation 

 of sporangia has been concluded, and the shedding of their contents has begun ; this 

 continues during spring. The old fronds, mainly occupied by broad riband-like sori 

 that have been rendered white by the shedding of their contents and by the trans- 

 parency of the medulla, are now rotten, and a moderate equinoctial gale suffices to 

 free the new young growths from their burdens 1 . 



Fig. 4S9. Microcystis pyrifera, (Turn.) Ag. Very much reduced. After Hooker and Harvey. 



The photic region extends to a depth of about 40 meters. The dysphotic 

 region possesses only a few stunted macrophytic Algae, but many Dia- 

 tomaceae. Where these cease and the aphotic region begins is at present 

 unknown. 



The cold temperate seas of the southern hemisphere possess a flora very 

 different from that of the northern seas. Sea-grasses (Zostera Mullen, Irm., 

 Z. Capricorni, Aschr., Z. tasmanica, Mart., Posidonia australis, Hook, f.), are, 

 it is true, common on the coasts of South Australia, Tasmania, and New 

 Zealand ; but they do not occur further south. Eucaceae, which, as regards 

 forms, attain their maximum development in the Australian seas, extend 

 further south (Auckland, Chatham Islands) with a small number of species; 

 the genus Fucus appears to be absent. The most striking species of Alga 

 of the southern temperate seas is Macrocystis pyrifera, (Turn.) Ag., which 



1 Kuckuck, II, p. 443. 



