368 ECOLOGICAL FACTORS, ETC. 



latter grows epiphytically on Nemalion helminthoides between June 

 and December. 



A word may conveniently be said here about the behaviour of 

 some species in relation to fish and fisheries (Savage, 1932). One of 

 the most outstanding examples is Phaeocystis pouchetii, a coloured 

 flagellate which, when present in quantity, gives the waters of the 

 North Sea a muddy appearance, the so-called ''baccy juice". 

 Herrings are repelled by this organism when it is present in mass, 

 and the vernal maximum of this organism off the Dutch coast turns 

 the northward herring migration west towards the coast of E. 

 Anglia and thus brings about the spring fishery (cf. fig. 209 A, B). 

 The occurrence of an abnormal autumn maximum out of its usual 

 station may completely change the grounds of the autumn fishery 

 during the southward migration: such an abnormal maximum is 

 known to have occurred in 1927 (cf. fig. 209 C). 



LIFE FORM 



A study of algal ecology leads one to the conclusion that the 

 distribution of the diflterent types appears to be largely controlled 

 by the nature of the habitat, e.g. rocky shore, sandy shore or salt 

 marsh, although of course there may be other factors because this 

 will not explain the predominance of the large kelps in the colder 

 waters and the predominance of the lime-encrusted forms in the 

 warmer waters. For this reason there would seem to be a need for 

 some sort of Life Form classification comparable to that of Raun- 

 kiaer's for the flowering plants. Such a system can be used to give 

 a quantitative picture of the composition of the vegetation and also 

 to demonstrate the absence of any type, thus raising the problem as 

 to why they are absent. Biological spectra, similar to those em- 

 ployed by Raunkiaer (1905), form a convenient way, if used with 

 caution, of comparing floras from two diflferent areas although they 

 are subject to the limitation that they do not indicate the dominant 

 types. 



Oltmanns' schema of 1905, which is one of the earliest, is 

 based largely upon morphological criteria, but in the light of 

 present knowledge it is more desirable to adopt a scheme with some 

 relation to habitat rather than one based on purely morphological 

 characters : 



