46 SUGARS 



of the Sugar Cane the juice is crushed out of the canes with 

 the help of rollers. In all cases the crude sugar is subjected to 

 subsequent processes of refinement. 



Fructose is most abundant in succulent fruits, and is an 

 important constituent of honey ; in both, however, it is mixed 

 with, usually smaller amounts of, grape and cane sugars. In 

 such cases the sugars are, of course, not of the nature of food- 

 reserves, but serve a biological purpose in connection with see'd- 

 dispersal and cross-pollination. 



All the four sugars above mentioned are found in foliage- 

 leaves, though in proportions that vary greatly both during the 

 day and night and at different seasons of the year. It is still 

 an open question whether glucose or sucrose is the first sugar 

 to be formed in photosynthesis ; but there is no doubt that the 

 other two, like the transitory starch, are secondary products. 

 Maltose appears to be produced invariably during the solution 

 of starch within the plant, and is consequently found not only 

 in foliage-leaves, but in germinating Barley (malt) and other 

 starch-containing seeds. The fructose found in leaves, on the 

 other hand, is formed by the breakdown of cane-sugar. 



The latter is readily split (in the presence of water, so-called 

 hydrolysis) into two molecules of monosaccharide by boiling the 

 solution with a few drops of some mineral acid (e.g. hydrochloric 

 acid) ; one molecule of glucose and one of fructose are obtained, 

 the mixture being known as invert sugar. Cane-sugar is similarly 

 converted into invert sugar by the agency of an enzyme invertase 

 found in most plants. In the same way the polysaccharides 

 above discussed can be split up with the formation of disaccha- 

 rides or monosaccharides, as the case may be. For instance, a 

 starch " solution " boiled with a few drops of a mineral acid 

 becomes clearer, and the ordinary reaction to iodine gradually 

 disappears ; the colour assumed with this reagent is now reddish, 

 owing to the presence of simpler polysaccharides known as dex- 

 trins. If the boiling be continued, the whole of the starch " solu- 

 tion" ultimately breaks down into simple glucose. Similarly 

 inulin gives rise to fructose. In the plant starch and inulin are 

 acted upon by ferments, diastase and inulase respectively, which 

 effect like changes, except that diastase breaks down starch into 

 the disaccharide maltose, which in its turn is acted upon by 



