POLYSACCHARIDES 85 



attributed to the action of pectase, but rather to the production of 

 insoluble jelly-forming substances from pectose or pectin by the 

 hydrolyzing action of the fruit-acids. The pectins are not hydrolyzed 

 by diastases, they are, however, hydrolyzed by special enzymes, the 

 Pectinases, found in malt and in certain moulds which liquify pectin 

 jellies with the production of reducing sugars. 



Coming now to those polysaccharides which are primarily of nutri- 

 tive importance, Starch is the form in which sugar is chiefly stored up 

 by plants for future consumption, although cane-sugar, inulin and 

 other carbohydrates frequently play a similar part. 



Starch is found in the greatest amounts in those portions of plants 

 which are subsequently to be drawn upon for the materials of growth. 

 Thus seeds, roots, bulbs, tubers and the pith of deciduous trees in 

 winter are particularly rich in starch, this carbohydrate frequently 

 comprising as much as eighty per cent, of the dry weight of the material. 

 The starch is stored up in these tissues in the form of stratified granules, 

 which differ characteristically in form and size in different plants. It 

 is by means of these characteristics of form, size and stratification of the 

 granules that we can tell very readily whether a starch alleged to have 

 been derived from one specified source has or has not been adulterated 

 in the pursuance of "legitimate business enterprise" with starch 

 derived from some other and cheaper source. 



The concentric rings, or stratifications of starch-grains represent 

 their gradual growth, and intimate that the growth of starch-grains 

 takes place rhythmically, periods of desposition alternating with 

 periods of rest. Starch is only slightly and very slowly changed by 

 cold water, but in hot water the grains swell up and finally burst, 

 forming what is known as "starch-paste." Neither starch nor starch- 

 paste reduces metallic oxides in alkaline solution. 



A very familiar test for starch is the formation of a very deep indigo- 

 blue coloration when it is acted upon by iodine solutions in the presence 

 of hydriodic acid or of an iodide. The color disappears on boiling and 

 reappears on cooling. In applying this test it is necessary to remember 

 that it is not given in the presence of excess of reagents which are 

 oxidized by iodine, such reagents, for example, as hydroxides of the 

 alkalies, or sulphurous or arsenous acids. It is in connection with 

 this test that we meet with very clear indications that starch is not a 

 homogeneous chemical unit, for varieties of starch are known which 

 do not give a blue, but a reddish-brown or a "port-wine" color with 

 iodine. We do not know to what these colorations are due, or whether 

 they are specific, i. e., yielded by one chemical individual alone, or 

 generic, i. e., yielded by a group of similar chemical individuals. 



On boiling starch with dilute mineral acids, glucose and only glucose 

 is obtained. Starch is therefore an anhydride of glucose. If the acid is 

 allowed only to act upon the starch in the cold, or with very gentle 

 heating, a modification of starch, known as "soluble starch" is obtained. 

 If we act upon starch for several weeks with cold dilute mineral acids, 



