390 PHYSIOLOGY 
more than one such crystal starts in the leucoplast, a compound or aggre- 
gate grain may result (fig. 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, a? 
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. 
In seeds of rice .... 68 In seeds of navy beans . 45 
In seeds of wheat .... 68 In seeds of flax .... 23 
In seeds of corn .... 60 In seeds of almond ... 8 
In seeds of pea .... 52 In tuber of potato ... 80 
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-celluloses. They consist often of mannans and galactans, 
which on digestion yield mannose and galactose, sugars that are quickly 
