CARBOHYDRATES ] 39 



Structurally modified sugars 



Rhamnose, a methylpentose, is frequently present as a part 

 of the glycosidic component of certain flavonoids. It does occur as the 

 free sugar, for example, in leaves and flowers of Rhus toxicodendron. 



Fucose, deoxygalactose, is found in the brown alga, Laminaria 

 digitata. Although botanical examples of the systematic utility of 

 fucose are lacking, there is an interesting example of genus specificity 

 involving fucose in the sea-urchin-egg jelly coat (Vasseur and Immer, 

 1949). The polysaccharide of Echinus esculentus jelly consists en- 

 tirely of galactose residues, that of Echinocerdium cordatum solely of 

 fucose residues, Strongylocentrotus draebachiensis, fucose with galac- 

 tose, and Paracentrotus lividus, fucose plus some glucose. 



Numerous other deoxy sugars, occurring often as steroid 

 glycosides, are known (Shafizadeh and Wolfrom, 1958; Reichstein, 

 1958). Deoxyribose is of ubiquitous occurrence in nucleotides of DNA. 



Branched chain sugars of higher plants are somewhat rare. 

 Apiose, illustrated below with hamamelose, are among authenticated 

 branched pentose and hexoses respectively: 



Hamamelose is found as a constituent of a tannin of Ham- 

 amelis virginica, but apiose has been identified as a free sugar in 

 leaves of the monocot, Posidonia australis (Potamogetonaceae). At 

 least it is the main sugar component following mild acid hydrolysis of 

 leaves, so it is to be considered as only conditionally a free sugar 

 (Bell et al., 1954). Previously apiose was known only as the parsley 

 flavone glycoside (the apioside of apigenin). 



Sugar alcohols 



(acyclic polyhydric alcohols) 



Relatively few sugar alcohols have been identified as naturally 

 occurring compounds. Usually the compounds are reported from a 

 rather diverse group of plants. For example, galactitol (dulcitol) is 

 found in certain red algae, fungi, and higher plants; sorbitol is found 

 in algae, monocots, and dicots; mannitol, which is quite widespread, 



