220 DISCOVERY REPORTS 



tude, for instance 20 or 30 . This is due to the fact that none of the more important 

 surface currents run directly east and west, all having some meridional component. 

 Reference to the chart in Fig. 1 will indicate clearly what is meant. Of the Atlantic 

 currents the warm Brazil current moving southwards along the eastern seaboard of 

 South America enables a large warm-water flora to be supported almost as far south as 

 50 S, while in the eastern Atlantic and Pacific are the cold upwelled waters of the 

 Benguela and Peru currents which support a flora which has a distinctly cold-water 

 facies. The disposition of the land-masses in the southern hemisphere and the currents 

 that operate around them have a marked effect upon the geographical range of the 

 various species. 



Apart from the main consideration of the oceanic currents which control to a large 

 degree the extent of the floras borne by them, the layering and zoning of the surface 

 waters, particularly in the South Atlantic, exert considerable influence upon certain 

 species. 



The analyses of a large number of stations have shown that it might be possible to 

 consider a third group, that is, the "Antarctic convergence flora", which is very abun- 

 dant in species and genera, and of course is an association of both cold-water and warm- 

 water floras. The consideration of the various floras must be in relation to the factors 

 that operate upon the species contained in them, that is, the floras are associations of 

 organisms acting and being acted upon at random by a set of chemical and physical 

 conditions, and it is likely therefore that modifications in structure, association and 

 habit will result from variation in the environmental factors that occur. In the main it 

 may be said that the warm-water flora is more weakly siliceous than that sustained by 

 colder waters, and further that the former is much richer in genera and species while the 

 latter is the richer in individuals. 



To consider the subsections a little more closely, the terms holoplanktonic, mero- 

 planktonic, etc., have been used more or less in the sense that Haeckel introduced them, 

 and in the sense in which they have been used subsequently by Ostenfeld and Lebour. 

 Under holoplanktonic are included the organisms which spend the whole of their time 

 as free-floating individuals. All truly oceanic species are holoplanktonic, and in certain 

 conditions several species which are regarded as belonging to the neritic flora are holo- 

 planktonic also. Tychopelagic species are those which spend most probably the greater 

 part of their life as bottom forms lying in mud or attached to a substratum, forming 

 ribbon-like bands and only entering the surface layers of the sea if torn forcibly from 

 their natural habitat by rough weather. Meroplanktonic species are littoral in so far 

 that they are associated with a coast-line. They are often chain-forming species which 

 may attach themselves to larger algae or rocks, but are never bottom forms, in the sense 

 that true tychopelagics are. Meroplanktonic forms are often found living under oceanic 

 conditions, and no sharp line of definition can be drawn between the two categories. The 

 division might be summarized by stating that all true oceanic species are holoplanktonic, 

 whereas the neritic flora may consist of several holoplanktonic species together with 

 meroplanktonic and tychopelagic ones. 



