i88 DISCOVERY REPORTS 



ported by the observations that these forms find their optimum in polar waters, under 

 conditions most closely approaching the littoral, i.e. along the ice-edge; and that they 

 still possess strongly silicified skeletons as instanced by their remaining recognizable in 

 diatom ooze. Certainly their spores seem to be carried by the ice, as Blessing found 

 long ago in the Arctic. Fluctuations in the strength of the surface current bringing ice 

 from the Weddell Sea up into the vicinity of South Georgia in spring will obviously 

 play a large part in determining the quantities of these forms present in the plankton. 

 Apart from the importance of surface currents as transporting agents, it may be 

 profitable to consider the possible influence of more deeply seated water movements. 

 It is well established that the Antarctic surface drift has a distinct northerly component, 

 and that when it sinks below the sub-Antarctic water at the convergence a tremendous 

 mortality amongst the phytoplankton ensues, instanced by the presence of abundant 

 diatom ooze in the bottom deposits in the vicinity. Doubtless eddies and the ice help 

 to maintain a number of spores within the Antarctic Zone, but it seems improbable that 

 the supply could be kept up by these means alone in the face of such an enormous drain. 

 This obviously applies with even greater force when animal organisms are considered. 

 A suggestion arising out of Hardy's observations on the rapid sinking of Coscinodiscus 

 bouvet to great depths, and from the work of Mr Fraser on larval Euphausians, shows 

 how this difiiculty may be overcome, but it should be understood that this explanation 

 is as yet hypothetical. Notwithstanding the great influx of Pacific water flowing east 

 into the Scotia Sea, which Clowes^ has recently demonstrated, there is little doubt that 

 the warm intermediate layer of water which is so marked a hydrological feature in 

 southern latitudes must have a well-defined southerly component. It replaces the water 

 flowing northwards at the surface and along the bottom. 



It is thus at least possible that resting spores derived from diatoms sinking near the 

 northern limits of the Antarctic Zone might ultimately be carried southwards through 

 the agency of this intermediate layer. On the analogy of the seeds of certain higher 

 plants, which require a long sojourn under uniformly low temperature conditions,^ 

 before they will germinate, there appears to be no physiological obstacle to some such 

 supposition. A few concrete facts also appear to support this hypothesis, notably the 

 occurrence of typically sub-Antarctic forms such as Rhizosolenia curva and R. poly- 

 dactyla to the south of the convergence, in dying condition, over the bank to the north- 

 west of South Georgia. If these forms had died and sunk to the north-westward in the 

 vicinity of the Antarctic convergence, and become involved in the warm intermediate 

 layer, this bank of comparatively shoal water, where violent vertical mixing is known to 

 occur, is just the position in which one would expect their dead frustules to come to 

 light. If dead diatoms properly belonging to other habitats can be brought into the 

 Antarctic Zone by this means, it is likely that Antarctic forms will be brought back also, 

 and may very probably form resting spores that survive. Another instance, from the 



1 Cp. Clowes, A. J., Nature, cxxxi, p. 189 (1933). 



2 The average temperature of the "warm" nucleus of the intermediate layer is ca. i-8" C. in the latitude 

 of South Georgia. 



