general notes 223 



(2) Cold-water flora 

 A. Oceanic. 



The holoplanktonic flora of the cold waters from oceanic habitats forms the major 

 part of the material examined in this work. Its composition might be said to consist of 

 a small number of discoid forms and a larger proportion of filamentous pennate forms. 

 In the main there is the enormous preponderance of the Corethron-Chaetoceros and 

 Fragilariopsis-Nitzschia associations which are so characteristic of the Southern Ocean 

 and associated seas. 



Owing to the cold Antarctic Drift which sweeps right across the South Atlantic from 

 Cape Horn past South Africa to the south of the Indian Ocean in an almost circumpolar 

 fashion, very few of the species from the warmer waters ever reach far south and become 

 established. Although cold-water species are found sometimes associating with a warm- 

 water flora, very seldom if ever is it found that warm-water species have become estab- 

 lished in a flora which is subject to polar or subpolar conditions. The normal habit of 

 the holoplanktonic discoid diatoms in cold water is solitary, and only under certain 

 conditions do they adopt colonial methods. These colonies are usually embedded 

 within a mucilaginous film, and are probably the result of some special form of repro- 

 duction by microspores. The chaetocerids are relatively small celled, bearing very 

 strongly developed appendages. The true solenoids, Rhizosolenia for example, are only 

 locally abundant, but when present are usually long and thin, with a few important 

 exceptions, e.g. R. rhombus and R. curvata. 



The two special groups included in the oceanic cold-water flora require further de- 

 finition, for in the strict sense of the term they are not truly holoplanktonic. Under the 

 heading parasitic, I refer to the diatom population which inhabits the skin of whales, 

 and although undoubtedly they could maintain their position in the surface layers of the 

 sea for a short time at least, they are by no means free-floating plankton forms. This 

 epizootic community Hart (1935) rightly has divided into two classes, viz. true con- 

 stituents of the skin film, and fortuitous species. Licmophora Lyngbyei is included in the 

 first class together with the specific forms of Cocconeis, and Hart pointed out that, al- 

 though the body of the whale does not provide an ideal substratum for the former 

 species, its extreme capacity for attaching itself to almost anything and establishing 

 colonies with great rapidity must account for its presence upon the skin film. This 

 property is of course the outstanding characteristic of the group of diatoms to which 

 Licmophora belongs, and is noticed equally strongly developed in the allied genus 

 Climacosphenia, which under subtropical conditions in Florida is regarded as a first 

 colonist. Of the fortuitous species upon the skin film, Hart is of the opinion that as they 

 include some of the dominant members of the neritic diatom flora they probably gain 

 access to the whales while they are awaiting to be flensed. The most noticeable fact con- 

 cerning the diatoms of the skin film is the entire absence of oceanic species, that is 

 species which are truly holoplanktonic. The entire skin flora is either tychopelagic or 

 meroplanktonic, most of them being stipitate epiphytes or bottom forms. 



