THE CHEMICAL CONSTITUENTS OF PLANTS 185 



The methyl pentoses.Fucose, C 5 H 9 (CH 3 )O 5 , has been obtained 

 from a sea- weed (Fucus nodosis). It occurs as its polymeride, 

 fucosan, in the cell walls of many sea plants. It is a crystalline, 

 sweet, very soluble substance, yielding methyl furfurol on distillation 

 with hydrochloric acid. 



Rhamnose, C-H 9 (CH 3 )0 5 .H 2 0, is not found in the free state in 

 plants, but occurs very frequently in compounds analogous to the 

 glucosides, from which dilute acids or alum solution set the sugar free. 

 As quercitrin it has been found in many plants, e.g., in sumach, hops, 

 tea-leaves, ash and horse-chestnut : 



C 21 H 20 O n + 2H 2 = C,H 14 0, + C 15 H ]0 7 . 



Quercitrin. Rhamnose. Quercetin. 



Many other glucoside-like bodies have been obtained from plants, 

 some yielding glucose as well as rhamnose. 



Ehamnose forms hard crystals, very soluble in water, has a sweet 

 taste, but leaves a bitter after-taste. It loses its water of crystallisa- 

 tion at about 105. By reduction it forms the penta-hydric alcohol 

 Ehamnite or Ehamnitol, C 6 H 14 O 5 , a sweet, soluble substance. 



Ehamnose, like glucose, is coloured yellow by alkalies and reduces 

 copper salts. It is coloured violet-blue by sulphuric acid and a-naph- 

 thol. 



2. The hexoses, C 6 H 12 O 6 . The hexoses occur widely distributed 

 in the vegetable kingdom, being found in the free state (as in ripe 

 fruits) or in combination with organic acids as glucosides. 



They are produced by the hydrolysis (i.e., the addition of the 

 elements of water) of the disaccharoses, polysaccharoses, or of the 

 glucosides. The hydrolysis is usually effected by unorganised ferments 

 or by the action of hot acids. 



They are generally crystallisable substances, soluble in water, pos- 

 sessing a sweet taste, and nearly insoluble in alcohol. They generally 

 exhibit optical activity, i.e., they rotate the plane of polarised light. 



They show the reactions of alcohols and ketones or aldehydes, and 

 possess the power, in presence of alkalies, of reducing cupric oxide to 

 cuprous oxide. With phenyl-hydrazine, H 2 N.NH(C H 5 ), the hexoses 

 yield, in presence of acetic acid, crystalline precipitates known as 

 osazones : 



C fl H 12 6 + 3HaN.NH.CgBL 

 = C H 10 4 .(N.NH.C 6 H 5 ) 2 + 2H 2 + NH 3 + NH.,C,H 5 



Glucosazone. Aniline. 



Many of the hexoses are capable of fermentation under the influ- 

 ence of yeast, Saccharomyces cerevisice, yielding alcohol and carbon 

 dioxide as the chief products. 



C 6 H 12 (i = 2C 2 H + 2C0 2 . 



Small quantities of amyl alcohol, C 5 H n OH, glycerol, C 3 H 5 (OH) 3 , 

 and succinic acid, C 2 H 4 (COOH) 2 , are also formed. Glucose, mannose, 

 d-galactose and ^-fructose are thus affected, while sorbose, gulose, 

 Z-fructose and the other hexoses do not ferment with yeast. 



