92 THE METABOLISM OF ALGAE 



Further information on the structure and nomenclature of 

 carbohydrates may be found in a number of books, e.g.^-^ 



Glucose or other reducing sugars occur in low concen- 

 trations in species of Chlorophyceae^^"''' ^'^^' ^^^ but are 

 present only in traces or are absent in demonstrable quantity 

 in representatives of the Bacillariophyceae,^*^ Phaeophy- 

 ceae,i2i, 127, 173 Rhodophyceae^^' i^s, i69 ^nd Myxophy- 

 ceae.^-^' ^'^ Free pentoses, like hexoses, are evidently of 

 little quantitative importance in algae. Mannitol, a hexa- 

 hydric alcohol which may be derived by reduction from 

 mannose or fructose, occurs in considerable quantity in 

 species of Phaeophyceae, in which it may sometimes form 

 more than 30 per cent of the dry weight. ^^ Mannitol is to 

 be found in only a few higher plants but appears to be of 

 general occurrence in the Phaeophyceae. ^^^' ^^^' ^'^ It is 

 absent from those representatives of the Rhodophyceae in 

 which it has been sought^^^ and does not appear to have 

 been recorded as occurring in any other algal class. 



CHoOH CH2OH 



I ' I 



HOCH HOCH 



I I 



CH 



O 



HCOH 



I 

 HCOH 



I 

 — CH2 



HOCH 

 I 

 HCOH 



I 

 HCOH 



CH2OH 



D-mannitol D-mannitan 



An anhydride of mannitol, mannitan, has been reported as 

 occurring in Pelvetia canaltculata.^^^ Dulcitol and sorbitol, 

 which are isomeric with mannitol, have been found in Bos- 

 trychia scorpioides and Iridaea laminarioides but not in a 

 numberof other species of Rhodophyceae.^^^' ^^^' ^^^ Flori- 

 doside, which is evidently glycerol a-D-galactoside, is 

 abundant and generally distributed in the Rhodophy- 

 ceae.^^' ^^"' ^^^ It is remarkable among naturally occurring 

 glycosides in belonging to the a-series. In many species of 



