438 THE ALGAE 



seasonal dimorphism is shown by the two morphologically similar 

 species Polysiphonia sertularioides and P. tenerrima, the former oc- 

 curring on exposed rocks from December to May whilst the latter 

 grows epiphj^cally on Nemalion helminthoides between June and 

 December. 



Life Form 



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

 tribution of the different 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 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 different areas although they 

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

 types. 



Oltmaim's scheme of 1905, which is one of the earHest, is based 

 largely upon morphological criteria, but in the light of present 

 knowledge it is more desirable to adopt a scheme with some rela- 

 tion to habitat rather than one based on purely morphological 

 characters : 



(i) Bush and tree forms (Bryopsis). 



(2) Gelatinous bush forms {Diatoms), 



(3) Whip forms (///maw/^a/Za). 



(4) Net forms {Hydrodictyon). 



(5) Leafy forms: (a) lattice (^^arwm), 



(h) ^2ig{Macrocystis\ 

 (c) buoy (Nereocystis), 



(6) Sack forms (Leathesia). 



(7) Dorsiventral forms {Delesseria). 



(8) Cushion, disc and encrusting forms {Ralfsia). 



(9) Epiphytes, endophytes and parasites. 



(10) Plankton. 



(11) Symbionts. 



