THE PRODUCTS OF METABOLISM I05 



by, (i) a tendency to form i : 3 linkages between mono- 

 saccharide residues rather than i : 4 Unkages such as are 

 found in other algal groups and higher plants and animals; 

 this is apparent in laminarin, floridean starch, agar and car- 

 ragheenin; and (2) a tendency to form polysaccharide sul- 

 phate esters as in fucoidin, agar and carragheenin. In view 

 of differences in morphology and in other chemical products, 

 this correspondence between the two classes cannot be 

 taken as evidence of phylogenetic affinity and perhaps only 

 reflects the similarity of the environments in which the two 

 types have evolved. Other noteworthy peculiarities of the 

 carbohydrate metabolism of these two groups are the for- 

 mation of a reserve polyglucose with /^-linkages in the 

 Phaeophyceae and the presence of an a-glycoside, florido- 

 side, in the Rhodophyceae. Lack of ability to synthesize 

 certain carbohydrates is characteristic of some classes. For 

 example, starch evidently does not occur in the Xantho- 

 phyceae, Chrysophyceae or Bacillariophyceae, and the same 

 three classes have a general tendency to store fats as reserve 

 materials rather than polysaccharides. 



These and other biochemical characteristics are sum- 

 marized in Table 9. It will be seen from this table and that 

 giving the characteristic pigments of the algal classes 

 (Table 2) that there are resemblances between certain of the 

 classes. These biochemical similarities, together with mor- 

 phological evidence, indicate an affinity between the Xantho- 

 phyceae, Chrysophyceae and Bacillariophyceae, which are 

 consequently grouped together as the Chrysophyta, and 

 between the Dinophyceae and Cryptophyceae, which are 

 grouped together as the Pyrrophyta.^^^ 



