NUTRITION 



391 



transformed into other compounds. The hemi-celluloses are especially 

 common in the endosperm of seeds, and are used as food by the embryo 

 in germination. They are deposited in layers on the interior of the cell 

 walls, sometimes to the great reduction of the lumen; yet through the 

 pits in the thickened walls the protoplast in each chamber maintains 

 communication by slender threads with its neighbor. This excessive 

 thickening imparts to such seeds a hornlike toughness, as in the coffee 

 " bean," or even a bony hardness, as in the date " stones." Sometimes 

 cotyledons and even bud scales have like deposits on their cell walls. 



Inulin. Inulin is comparatively restricted, being characteristic of 

 a few large families (and occasional elsewhere). It occurs dissolved in 

 the cell sap, especially of subterranean organs. It is a very complex 

 carbohydrate, though less so than starch, having a formula w(C 6 H 10 O 6 ), 

 where n is probably as much as 12 or 18. Whereas starch is built from 

 glucose units, inulin is formed by the condensation of fructose units, 

 and is comparable in complexity with some of the dextrins, which starch 

 yields by digestion. When inulin-containing tissues are put into strong 

 alcohol, the inulin is deposited as spherites (see Part III, fig. 1209). 



Fats. Fats are among the most important and valuable of surplus 

 foods. In most plants they exist as small drops of oil in the protoplast; 

 but in some cases, as in cacao, they are solid at ordinary temperatures. 

 The most universal storage place for fats is the seed, where it is in some 

 cases the dominant form of food, and in almost all it is present in greater 

 or less quantity. It is by no means confined to seeds, but occurs in the 

 flesh of fruits (olive), in rhizomes (potato, iris, and sedges), in bulbs 

 (onion), and in roots (carrot). In almost every part of a plant, indeed, 

 small quantities of oil may be found, and from many reservoirs it can be 

 extracted in commercial quantities. 



True oils must be distinguished from volatile or essential oils, which are common 

 in leaves and flower parts. The latter usually have a distinct odor and make a 

 temporary translucent spot on writing paper, whereas that made by true oils is 

 lasting. 



Accumulated oils are obtained for commercial uses by crushing and pressure; 

 but as only a portion of the oil (which forms 2 to 68 per cent of the dry weight) can 

 be recovered thus, the " cake " remaining, with its residue of oil and other sub- 

 stances, may still be valuable food for animals, as is the case with cotton and flax 

 seed. 



Proteins. Proteins, unless they take on a specific solid form, cannot 

 readily be distinguished from resting protoplasm. Thus, the " gluten " 



