178 STORAGE OF FOOD AND WATER 



is at its maximum in ordinary stems and roots, and in special 

 storage organs. The largest percentage of stored starch occurs, 

 of course, in dry seeds, such as those of the cereals and legumes. 

 Thus the potato is 20 per cent, starch, while beans contain 45, 

 peas 58, oats 47, barley 48, rye 60, wheat and millet 64, maize 65, 

 and rice 76 per cent, of starch. 



Dextrose, Lcevulose, and Saccharose. Dextrose (glucose or 

 grape sugar) is the commonest form in which the non-nitroge- 

 nous foods circulate throughout the plant. It is probably the 

 form in which food is first made by the chloroplasts, and it 

 arises secondarily by the digestion of starch, fatty oils, inulin, 

 cellulose, cane sugar, etc. It occurs in solution in the cell-sap, 

 and when the sap is evaporated it is thrown down in the form 

 of crusts or warty agglomerations of pseudo-crystals. Small 

 percentages of dextrose frequently occur in storage tissues and 

 sometimes in association with laevulose and saccharose. Dex- 

 trose and laevulose (fructose or fruit sugar) are the sugars in 

 sweet fruits, in the nectar of flowers, and in the bulbs of Allium 

 cepa and Ornithogallum arabicum, and in the underground 

 parts of species of Primula and Globularia. 



Saccharose (cane sugar, beet sugar) occurs as reserve food 

 in the maples, sugar- and sorghum-canes, beet-root, etc. Sugar 

 maple sap yields somewhat less than 3 per cent, of sugar, sugar- 

 cane sap about 18 per cent., sugar-beet sap 16 per cent., and 

 sorghum-cane about 14 per cent, of saccharose. Cane sugar 

 also occurs in some fruits such as the banana and pineapple. 



Fatty Oils and Fats. Fatty oils are fluid at ordinary tem- 

 peratures and the fats are solid. They are, of course, not soluble 

 in, nor miscible with water or the cell-sap; and in the plant 

 cell the fatty oils occur in a very fine emulsion, and the fats in 

 groups of exceedingly minute crystals throughout the meshes 

 of the cytoplasm. 



The fats and fatty oils as they occur in plants are mixtures 

 of glycerine esters or glycerides of palmitic, stearic, and oleic 

 acids; and the palmitic ester is called palmitin, the stearic, 

 stearin, and the oleic, olein. Palmitin and stearin are solid at 



