60 C. HEDLEY. 



plant marks the level of constant submergence; on floating 

 pontoons where the range of the tide is artificially abolished, 

 the dwarf kelp is attached just below the surface. After a 

 gale the dwarf kelp wrenched off the rocks by the storm, is 

 stacked in piles on the beaches. The chemical composition 

 of Eklonia exasperata was studied by O. J. White. 1 The 

 Eklonia is orange-brown in colour, and contrasts with the 

 more ye] low Sargassum with which it associates. 



The latter genus is represented on this coast by about 

 twenty species, one of which, S. tristichum (fig. 24) from 

 Balmoral Beach is here illustrated. 



A regular transition from wealth to poverty of seaweeds 

 occurs as an observer travels from temperate to tropical 

 beaches. In the latitude of Sydney there is still a consider- 

 able development, as is shown by the luxuriance of the 

 Hormosira (Plate II, fig. 4) on the flat near Long Reef. 



It was suggested, but the idea was not convincing, that 

 the dearth of the sea-weeds in the tropics, was due to the 

 heat of the sun scorching the vegetation at low tide. 

 Recent bacteriological research offers another explanation. 

 A marine bacillus, Pseudomonas calcis, abundant both in 

 the mud and surface water of West Indian seas, was dis- 

 covered by Drew 2 to have the curious property of abstract- 

 ing nitrogen from sea water. It is probable that this or 

 similar species have a wide distribution in tropical seas 

 and reach Australia. So that starvation by denitrifying 

 organisms may cause that poverty in sea weeds of tropical 

 beaches on which several observers have remarked. 



The upper zone of the ocean reef is bare of visible vege- 

 tation. Climbing higher than its fellows Tectarius (fig. 4) 



1 White, this Journal, xli, 1907, p. 95. 



2 G. H. Drew, Publication No. 182, Carnegie Institution of Washington, 

 1914, pp. 7-45; Kellerman and Smith, Journ. Washington Academy of 

 Sciences, iv, 1914, pp. 400—402. 



