THE PRODUCTS OF METABOLISM 93 



Ceramiales, Gigartinales and Cryptonemiales, sodium 

 mannoglycerate is more abundant than floridoside.^"'' 



HO 



Floridoside 



Among oligosaccharides, sucrose occurs in appreciable 

 quantities in Chlorella and Scenedesmus^^ but does not 

 appear to have been found in more than traces in represen- 

 tatives of classes of algae other than the Chlorophyceae. 

 Trehalose, i -[a-D-glucopyranosido]-a-D-glycopyranoside, 

 which is commonly to be found in fungi but not in higher 

 green plants, is present in various species of Myxophy- 

 ceae^^^' ^'^ and in freshwater species of Rhodophyceae.^® 



Glucose polymers form the principal intracellular reserve 

 products in several classes of algae. The starch of the 

 Chlorophyceae stains blue or purple with iodine and appears 

 to be essentially similar to that of higher plants. Starch 

 contains two principal constituents, i.e. amylose, which 

 consists of long unbranched chains of a-D-glucopyranose 

 units united by i : 4 linkages, and amylopectin, also con- 

 sisting of a-D-glucopyranose units but having branched 

 chains. The starch of Polytomella caeca contains 84 to 87 

 per cent of amylopectin. ^^ Floridean starch, which occurs 

 in the form of small grains staining brown with iodine or, 

 on swelling, violet, is the characteristic reserve product of 

 the Rhodophyceae.^^^ It consists entirely of glucose residues 

 but is structurally different from normal starch in contain- 

 ing a large proportion of i : 3 linkages and is resistant to 

 attack by /5-amylase.^* Paramylum (paramylon), the charac- 

 teristic reserve product of the Euglenineae, does not stain 

 with iodine but yields mostly glucose upon hydrolysis^ ^^ 

 and is evidently related to starch. A starch-like product 

 giving a blue colour with iodine either directly^^^ or on 



