128 PHAEOPHYCEAE 



II. Polystichineae. Plants built up by intercalary growth into 



a parenchymatous thallus, e.g. Punctariaceae, Dictyo- 



siphonaceae, Laminariales. 



(c) Cyclosporeae. Plants possessing a diploid generation only, 



e.g. Fucales. In view, however, of the most recent interpretation 



of the life history of the Fucales (p. 189) the Cyclosporeae should 



now be classed with the Heterogeneratae, division Polystichineae. 



It would seem impossible to construct a classification of the 

 Phaeophyceae on a satisfactory phylogenetic basis because they 

 would appear to have diverged and converged greatly during the 

 course of evolution. As a group they are very widespread and are 

 confined almost entirely to salt water although Pylaiella is some- 

 times found in brackish water and Lithoderma in fresh water. 

 Some of the species commonly exhibit morphological variations 

 and it has been shown that these may depend on [a) season of the 

 year, and {b) nature of the locality. Church (1920) has given us an 

 elaborate account of the morphology of the Phaeophyceae, and he 

 suggested that if a brown flagellate came to rest it could develop in 

 one of three directions to give : 



{a) Uniseriate filaments which occupy a minimum area and 

 obtain maximum light energy per unit of area, growth being either 

 distal or intercalary. 



(6) A mono- or polystromatic thallus which occupies a maximum 

 area and obtains a minimum light energy per unit of area. 



{c) Mass aggregation. 



A morphological examination of the brown algae will show that 

 development has taken place along each of these directions, often 

 resulting in plant bodies of a complex construction, and the 

 following types can be recognized among the various species : 



{a) Simple filaments (e.g. Acinetospora). 



(b) Branched filaments (e.g. Pylaiella). 



(c) Erect filaments arising from a basal portion (e.g. Myrionema). 



(d) Interwoven central filaments (cable type, e.g. Mesogloia). 

 {e) Basal portion only {reduced filamentous or cable type, e.g. 



Phaeostroma). 



(/) Filaments uniting to form a sphere {hollow parenchymatous 

 or modified cable type, e.g. Leathesid). 



{g) Multiseptation of primary cable type (e.g. Chorda). 



