274 DISCOVERY REPORTS 



certain 'species' together (e.g. all previously described species of Corethron as 'phases' 

 of C. criophilum Castracane, which is called the 'type phase'). This use of the more 

 general term ' phase ' to describe subspecific rankings, previously labelled ' varieties ' and 

 'forms' in rather indiscriminate fashion, wherever a clear sequence of intermediate 

 stages can be shown to exist, seems logical and is very useful in practice. As the first 

 clear acknowledgement by a recognized taxonomic expert of the extreme variability of 

 plankton diatoms, it is particularly encouraging to the unfortunate plankton worker who 

 is continually grappling with problems presented by this exasperating property. 



In his notes on the divisions of the flora, Hendey is upon less certain ground, owing 

 mainly to the limited amount of material he examined (1937, pp. 163-99). Two hundred 

 and twenty odd stations distributed over all the regions visited by the Discovery in- 

 vestigations from 1927 to 1935 may well have been ample for systematic revision, but 

 quite obviously preclude the possibility of considering the seasonal variation in any one 

 area, and it is well known that the quality of the phytoplankton varies very considerably 

 with the seasons, except in some tropical seas. 



The broad division of the flora into cold- and warm-water species, with a dividing 

 line mainly coincident with the subtropical convergence but otherwise based on un- 

 specified thermal considerations, is too wide to be of any assistance in considering 

 conditions within the Antarctic zone, and ignores the cosmopolitan distribution of some 

 important species. It is chiefly for these reasons that Hendey's table (1937, pp. 226-7) of 

 ' species typical of the cold-water flora ' shows some marked differences from my own 

 findings, though the disagreement is far less marked when one considers his distri- 

 butional notes on individual species. 



It is very interesting to note that Hendey has experienced the same difiiculty in the 

 precise application of the HaeckeUian terms 'oceanic', ' meroplanktonic ', etc. (p. 220) 

 that I have already had occasion to mention. This again may cause apparent rather than 

 real differences between our findings. The difficulty arises because we have only cir- 

 cumstantial evidence as to whether the majority of plankton diatoms are meroplanktonic 

 or holoplanktonic, using the words in the strict sense. In the northern hemisphere work 

 on the phytoplankton has been going on so much longer and more intensively that we 

 may safely regard the accumulation of this evidence as sufficient to be conclusive for 

 most species. In the far south it is still necessary to proceed with caution. Conditions 

 are further complicated by pack-ice maintaining a small proportion of meroplanktonic 

 forms in the open ocean at the greatest possible distances from land, which may 

 flourish for a time among the truly oceanic species after the ice has dispersed. Yet again 

 many forms that appear to be truly oceanic still reach their maximum abundance in 

 neritic areas. Hence Hendey's tabulation of some species as both holo- and mero- 

 planktonic, oceanic and neritic, is not so paradoxical as it appears at first sight. 



My object in pointing out the following important differences between my findings 

 and those expressed in Hendey's table (pp. 226-7) of ' species typical of the cold-water 

 flora' is to avoid possible misunderstanding in the future. It must be realized that I 

 have the advantage of much more numerous observations, many on material obtained 



