136 PLANT PHYSIOLOGY 



rotary, the mixture is commonly called invert sugar. Unlike 

 glucose, levulose is soluble in alcohol and is a ketone instead of 

 an aldehyde. About a fourth sweeter than cane sugar, it is the 

 sweetest of the ordinary sugars. Jerusalem artichokes yield on 

 hydrolysis 10-15% of levulose and now form the chief com- 

 mercial source of this sugar. 



Mannose does not occur free in plants, but is obtained from the 

 hydrolysis of its condensation products, known as mannans or 

 mannosans. It is obtained from the sap of the manna ash (Fraxinus 

 ornus) and from the hemicelluloses of peas, coffee beans, date 

 stones, and vegetable ivory nuts (Phytelephas) . 



Galactose occurs rarely, if ever, free in plants, but is a constitu- 

 ent of the trisaccharide, raffinose, and of the glucoside, digitalin. 

 It is also commonly found associated with the pentoses in gums 

 and mucilages. Agar-agar, which is a mucilage obtained from the 

 red algae, yields a high percentage of galactose, and the pectins of 

 carrots and pears are also rich in this sugar. In the animal king- 

 dom it occurs as one of the constituents of milk sugar, which 

 consequently on hydrolysis yields galactose. It is only slightly 

 sweet, but otherwise it resembles glucose in many of its properties. 



Sorbose comes from the juice of the fruit of the European moun- 

 tain ash (Sorbus aucuparia). It probably does not exist as such 

 in the plant but is rather an oxidation product. It is not ferment- 

 able by yeast, but in other respects it much resembles fructose. 



Disaccharides. — The disaccharides are formed by the union 

 of two monosaccharides with the elimination of water: 



2C6Hi206=Ci2H220n + H20. 



They might be formed in one of three ways: (1) by linking the two 

 alcohol ends of the molecules together, (2) by linking the two 

 aldehyde ends or the aldehyde and ketone, or (3) by linking the 

 aldehyde (ketone) of one with the alcohol of the other. If an 

 aldehyde group is still free, as would occur in the first and third 

 cases, the sugar is a reducing sugar; otherwise it is not. When 

 acted upon by dilute acids, specific enzymes, or other hydrolyzing 

 agents, the disaccharides are broken down into their component 

 monosaccharides. 



Reducing Sugars. — Maltose, also known as malt sugar, is found 

 widely distributed in plant tissues although in small quantities. 

 It is formed from the hydrolysis of starch, and is obtained in 



