MICROPLANKTON 207 



Sverdrup et al.) has added approximate size-limits — 60-1100/^ — for the microplankton. Now since 

 Ekmann is dealing with zooplankton this may suit his purpose well enough ; but many phytoplankton 

 organisms, especially colonial diatoms, are well sampled by nets, although the individual cells are well 

 below 60 n in greatest dimensions. The colonial habit permits their retention by the silk, while their 

 highly developed powers of flotation lead to differential settling-rates or actual failure to settle, 

 defeating the centrifuge or reversing-microscope methods of estimating relative abundance, that 

 would otherwise be far more satisfactory than the old net method. The postulation of actual size- 

 limits also brings us up against the fact that in Nature individual species can always be found whose 

 normal range of size-variation bestrides any proposed limit. Many individual species of diatoms and 

 of dinoflagellates normally vary in size from about 35 /i to over 100//, and a single important example 

 of a species completely overlapping the proposed upper limit is provided by the well-known Noctiluca 

 tniliaris. Most samples of this cosmopolitan organism that we have examined, from many parts of 

 the world, have had the majority of individuals ranging between 300 and 600// in greatest dimensions, 

 but some frequently attain to 1200 or 1500// and have been known to reach 2000// (Lebour, 1925, 

 p. 69). Clearly we must admit with Gran (in Murray and Hjort, 1912) and with Johnstone, Scott and 

 Chadwick (1924, p. 75) that ' ... it is impossible to establish clear and absolutely logical distinctions . . . ' 

 in the application of these broadly classificatory terms. 



This becomes even more obvious when other technical terms, broadly descriptive of aetiological 

 attributes of various categories of plankton organisms, such as those introduced by Haeckel and the 

 Kiel school of planktologists, and by other workers in this field from many countries, come under 

 review (cf. Hart, 1942, p. 268). They are but a part of the jargon without which generalizations cannot 

 be succinctly expressed, and attempts to define them with undue regard to a rigid specialized connota- 

 tion lead to frustration, because Nature herself seems able to find ' exceptions to every rule '. This 

 obtrusive antithesis should not mean that naturalists must foreswear the ecological point of view. 

 Major types of woodlands, whether defined by foresters or (in somewhat different terms) by terrestrial 

 plant ecologists, are none the less real entities because some kinds of trees are to be found in more than 

 one of them. 



This problem of terminology may be partially solved by the growing practice of holding international 

 conferences of recognized leading workers in a given subject, to seek agreement on definitions accept- 

 able to the majority. The alternative is for the individual worker to redefine his own usage when 

 necessary. 



The first solution is hampered by language difficulties and the rank growth of 'the literature', 

 such that the richest of languages proves insufficient to provide suitable terms that can be used in 

 specialized connotation, without trespass upon the jargon of some other ' branch of knowledge '. The 

 second, to some extent inescapable, can easily lead one into tedious and unprofitable etymological 

 discussion ! 



Here we attempt a working compromise by following the usage of Sverdrup, Johnson and Fleming 

 (1946) for most ecological terms. This usage is mainly derived from that of the earlier works 

 mentioned above. When less widely known terms have been employed we have tried to make their 

 meaning plain from the context where they first appear. The meaning of some phrases coined merely 

 for description of the present data will, we hope, prove sufficiently clear if the words are read in their 

 generally accepted, non-specialized sense. 



Terms relating more particularly to the phytoplankton are used in the sense that Gran and Braarud 

 (1935) have ascribed to them, as nearly as the differing conditions in this part of the southern hemi- 

 sphere permit. They have been evolved from the earlier system of ' plankton elements ' devised by 

 Gran (1902, 1932) for ecological characterization of the species, during the period when plankton 



