6 THE ALGAE 



those present in the higher plants and, furthermore, they are 

 present in much the same proportions. 



The Characeae comprise one predominantly fresh-water order, 

 the Charales, which differ from other green algae (Chlorophyceae) 

 because of their remarkable morphological and reproductive fea- 

 tures. They are consistently uniform in structure with characteris- 

 tic nodes and internodes and whorls of branches arising at the 

 nodes. There is no asexual reproduction and the sexual reproduc- 

 tive organs present an elaborate oogamy. There is some evidence 

 for beheving that the plants may have a common ancestry with the 

 Chaetophorales (seep. 115). 



(2) Phaeophyceae 



This group comprises the common brown algae of the seashore, 

 and it is worth noting that the majority are wholly marine, pre- 

 dominantly of cold waters. There are a few, rare fresh- water 

 species. The brown colour is due to the fucoxanthin pigment 

 which masks chlorophyll a (see Table i). The principal product of 

 photosynthesis is a sugar alcohol, known as mannitol, together with 

 the polysaccharide laminarin, whilst the cell walls not only contain 

 cellulose but also specific materials such as algin and fucoidin (see 

 p. 452). In some of the larger and more advanced brown algae these 

 compounds are present in sufficient quantity to be of commercial 

 importance (see p. 446). The class is further characterized by the 

 fact that the sugar residues tend to have a i : 3 linkage instead of 

 the more normal i : 4 linkage between the carbon atoms. The 

 simplest forms are filamentous, and there are all stages of develop- 

 ment and increasing differentiation up to the large seaweeds of the 

 Pacific and Arctic shores with their great size and complex internal 

 and external differentiation. There is also a number of greatly 

 reduced parasitic and epiphytic forms. The pyriform, motile repro- 

 ductive cells, which possess two flagella, one directed forwards 

 and the other backwards, are commonly produced in special organs 

 or sporangia that are either uni- or plurilocular. Some members 

 possess non-motile, asexual reproductive spores (monospores and 

 tetraspores). Sexual reproduction ranges from isogamy to oogamy, 

 but in the latter case the ovum is liberated before fertiUzation. The 

 life cycles are extremely diverse, and although alternation of 

 generations does occur, it is an irregular rather than a regular 

 process. 



