39° PHYSIOLOGY 



more than one such crystal starts in the leucoplast, a compound or aggre- 

 gate grain may result (lig. 662). The grains may show irregular layers 

 (fig. 660), this appearance signifying differences in the proportion of 

 water, composition of material, etc., doubtless determined by variations 

 in the available sugars and other conditions during the growth of the 

 grain. 



The starchy reservoirs are sources of important foods for men and animals, as 

 well as plants. Many of our farm and garden crops are such storage organs, 

 greatly improved and enlarged by breeding. Potatoes, sweet potatoes, yams, all 

 the cereals, peas and beans, arrowroot, sago, and tapioca are widely used plant 

 products, whose most abundant constituent is starch. The extraction of starch for 

 commercial purposes, especially from potatoes and corn, is an industry of consider- 

 able magnitude, as is also the production of alcohol by the fermentation of glucose 

 derived from the starch of these plants. The following table shows the approxi- 

 mate starch content of some common food reservoirs, in percentages of their dry 

 weight. 



Sugars. — The chief storage form of the sugars is saccharose, or cane 

 sugar. While glucose and fructose may be counted as constituents of 

 almost every active cell, they do not accumulate in nature to any great 

 extent, whereas saccharose in some plants, such as sugar cane and beet, 

 is almost the only form of surplus food, and in many it accompanies the 

 reserves of starch. The commercial supply of sugar is obtained chiefly 

 from cane and beet, while sorghum, maple, and certain palms furnish 

 a relatively small or local supply. 



Sugar is extracted from cane by crushing and washing, clarifying the liquor 

 and concentrating it. Beets are finely sliced and the sugar is extracted by diffusion, 

 then recovered by clarification and concentration of the solution. The cultivated 

 races of beet now average nearly 15 per cent of sugar, with some samples going 

 over 20 per cent, as against less than 7 per cent when breeding began. Cane juice 

 yields 10-18 per cent, and maple sap 2-5 per cent of saccharose. The refining of 

 sugar by redissolving and purifying removes the coloring and flavoring matters 

 which give to crude sugars from different plants their distinctive taste. 



" Reserve cellulose." — This name has been applied to food accumu- 

 lated upon the walls of cells; yet the substances are quite different from the 

 cellulose which forms the permanent part of the wall, and should rather 

 be called hemi-celluJoses. They consist often of mannans and galactans, 

 which on digestion yield mannose and galactose, sugars that are quickly 



