RHIZOSOLENIA CURVATA 445 



in the work described in this paper is accepted, a question of great interest immediately 

 presents itself. How is this passively drifting holophytic organism able to maintain itself 

 within the narrow limits of the colder portion of the sub-Antarctic Zone, where it ap- 

 parently finds its optimum? That some of the individuals temporarily driven into the 

 Antarctic Zone eventually find their way back again is highly probable, for the surface 

 drift is known to have a northerly component. It is the limited distribution of the species 

 to the north of the Antarctic convergence that demands explanation, for the main trend 

 of the surface drift, apart from local interruptions such as have been described, has a 

 northerly component in the sub-Antarctic Zone also. We have seen that the organism 

 is sometimes carried far south of its proper habitat by subsurface transportation in the 

 warm deep water. This, however, could not help to maintain it in its ideal environment, 

 owing to the much greater depth at which the warm deep water lies in the sub-Antarctic 

 Zone, and in the absence of land masses in the convergence region to cause upwelling 

 on a large enough scale to complete the return journey by such an agency. Eddy action 

 in the turbulent region of mixed surface water immediately to the north of the con- 

 vergence may well provide a partial explanation, but does not account for the regular 

 occurrence of the species in relatively large numbers more than ioo miles north of the 

 convergence and its absence in the northern part of the sub-Antarctic Zone. 



An adequate answer to the question appears to me, however, to be supplied by 

 Deacon (1933, p. 207). He has been able to demonstrate that in the South Atlantic not 

 only does the sub-Antarctic surface water tend to sink in about 45 S, but that there is a 

 shallow southward-flowing subsurface current which would readily mix with the surface 

 layers on reaching the turbulent areas immediately to the north of the Antarctic con- 

 vergence. A hydrological system such as this exactly fits in with the known distribution 

 of R. curvata in the southern ocean, if we are permitted to assume that, like other 

 members of the genus, it is able to survive for considerable periods below the photic 

 zone in a resting condition. 



If the probable value of R. curvata as an indicator of the extent of sub-Antarctic in- 

 fluence to the south of the average position of the convergence is admitted, it appears 

 that it is in the region of the S -shaped bend in the Scotia Sea that the position of the 

 Antarctic convergence is most variable, and that it is there that mixing across the con- 

 vergence takes place most frequently. 



REFERENCES 



Deacon, G. E. R., 1933. A general account of the Hydrology of the South Atlantic Ocean. Discovery Reports, 



vii, pp. 171-238. 



1937. The Hydrology of the Southern Ocean. Discovery Reports, xv, pp. 1-124, pis. i-xhv. 



Gran, H. H., 1912. Pelagic Plant Life. In Murray and Hjort, "Depths of the Ocean". Pp. i-xx, 1-821, 



maps i-iv, pis. i-ix. London. 

 Hardy, A. C, 1935. Phytoplankton. In Hardy and Gunther, The plankton of the South Georgia whaling 



grounds and adjacent waters, 1926-7. Discovery Reports, XI, pp. 1-456. 

 Hart, T. John, 1934. On the Phytoplankton of the South-west Atlantic and the Bellingshausen Sea, 1929-31. 



Discovery Reports, vm, pp. 1-268. 



