4 CLASSIFICATION 



wholly confined to the sea. The brown colour is due to the presence 

 of a pigment, fucoxanthin, which masks those other chlorophyll 

 constituents which are present. The products of photosynthesis are 

 alcohols, fats, polysaccharides and traces of simple sugars so that 

 there is evidence of some diversity of metabolism. The simplest 

 forms are filamentous, and there are all stages of development and 

 increasing diflferentiation up to the large seaweeds of the Pacific and 

 Arctic shores with their great size and complex internal and external 

 diflferentiation. The motile reproductive cells, which possess two 

 flagellae, one directed forwards and the other backwards, are 

 commonly produced in special organs or sporangia that are either 

 uni- or plurilocular. Sexual reproduction ranges from isogamy to 

 oogamy, but in the latter case the ovum is normally liberated before 

 fertilization. The life cycles may be extremely diverse and are 

 perhaps better regarded as race cycles (cf. p. 246). 



(9) Rhodophyceae. The members of this class form the red 

 seaweeds, and although most of them are marine nevertheless a 

 few are fresh-water. Their colour, red or bluish, is caused by the 

 presence of the pigments phycoerythrin and phycocyanin, whilst 

 the product of photosynthesis is a material known as "floridean 

 starch". Reproductive stages with locomotor appendages are not 

 known, even the male reproductive body being without any organ 

 of locomotion. The simplest members are filamentous, and again 

 all stages of diflFerentiation up to a complex body can be found, 

 although they do not develop to quite the same degree of complexity 

 as the Phaeophyceae. Very obvious protoplasmic connexions can be 

 distinguished between the cells of nearly all forms except in the 

 small group known as the Proto-florideae (cf. p. 217). Sexual 

 reproduction is oogamous, the ovum being retained upon the 

 parent plant, and although the subsequent development of the 

 zygote is varied to a certain extent, it usually gives rise to filaments 

 which bear special reproductive bodies or carpospores, and these 

 latter are responsible for the production of the tetrasporic diploid 

 plant. Most of the members exhibit a regular alternation of genera- 

 tions. 



(10) Nematophyceae. This is a fossil group of which one genus 

 has been known for a long time (Nematophyton) whilst the other 

 has only recently been described (Nematothallus). There is still 

 considerable doubt as to their true affinities, but it would seem that 



