98 THE METABOLISM OF ALGAE 



from many Phaeophyceae, e.g. Fiicus spp. and Laminaria 

 cloustoni. The principal constituents of this polysaccharide 

 are L-fucose (a methyl pentose) residues esterified by sul- 

 phuric acid. These residues are evidently united chiefly by 

 I : 2-a links; for a possible constitution see p. 97.^-^' ®^ 



There is some evidence that chitin, which is composed 

 of N-acetylglucosamine residues and which is a charac- 

 teristic component of the cell wall of fungi and of the 

 exoskeleton of insects, occurs in the cell walls of certain 

 species of Xanthophyceae.^^^" 



Many other substances of a carbohydrate nature, variously 

 described as pectins, mucilages and hemicelluloses, are to 

 be found in algae but have not as yet been adequately 

 characterized (see refs. 106, 107, 221, 117, 169, 170). 



LIPIDES 



Table 7 summarizes results of fatty acid analyses of vari- 

 ous algae. These results are in general agreement with those 

 of less extensive analyses made on other algae representative 

 of the same classes. ^^^' ^^' ^^" 



The Cjg acid, palmitic acid, appears to be the most gener- 

 ally abundant of the saturated acids. In algae of all classes, 

 unsaturated acids predominate, principally those having 

 16, 18 and 20 carbon atoms. An exception to this is found 

 in the fructifications of higher marine algae such as Alaria 

 crassifolia, of which over 50 per cent of the fatty acids are 

 saturated. 2' 9 Having regard to the considerable variations 

 in the composition of the fatty acid fraction that may occur 

 in a single species (see p. 118) it does not appear that there 

 are any consistent differences between the fatty acids pro- 

 duced by algae of different classes and higher plants growing 

 in similar habitats. 



The fatty acids of Chlorella sp., Scenedesmus sp. and 

 Nitzschia palea are present as triglycerides^^' but it has 

 been reported that freshly collected marine diatoms, e.g. 

 N. closterium, may contain considerable proportions of free 

 fatty acids, apparently irrespective of species. ^^ An explana- 

 tion of this difference has not yet been put forward and 



