FRUITS AND SEEDS AND THEIR USES 291 



in such cases the seed is ustially protected by a specially hard 

 layer that belongs to the fruit coat. 



294. Kinds of Food Found in Seeds. — Many seeds are 

 valuable to us because the food stored in them for the use 

 of their embryos is in a form that we can easily use. The 

 foods upon which the human race most largely depends are 

 obtained from the seeds of three of the cereal grains — wheat, 

 Indian com, and rice. Many other seeds, too, such as those 

 of other cereals, nuts, beans, peas, and lentils, though used 

 to a less extent, are nevertheless very important sources of 

 himian food. Starch is by far the commonest form in which 

 food is stored, both in seeds and in other parts of plants. 

 It is starch that is present in largest amount in the seeds of the 

 cereal grains, with the exception of the sweet corns. Starch 

 is a convenient form of storage food because it is dry and 

 compact, so that much can be stored in a comparatively small 

 space, and because, although it will not itself dissolve in 

 water, it may be easily and quickly changed to a substance 

 (a sugar) which dissolves readily. Starch is a carbohydrate. 

 Some other carbohydrate foods are found in seeds ; especially 

 some of the sugars (as in sweet corns) and cellulose, the sub- 

 stance of which cell walls are largely made. The cells of the 

 endosperm of some seeds (for example, those of the date, 

 the onion, and asparagus) have thick walls, and when the 

 seeds germinate some of the cellulose of the endosperm cell 

 walls is changed b}' means of an enzym to sugar, which is 

 then used by the growing embryo. 



Besides carbohydrates, the most important classes of food 

 substances found in seeds are fats and proteins. Fats are 

 present in small quantities in the seeds of cprn and of many 

 other plants. Some seeds contain large amounts of fats ; 

 examples are the walnut, the hickory nut, and the peanut, 

 the seeds of the flax, from which linseed oil is made, those of 

 the cotton, and those of the castor-oil bean. Proteins are 

 present in small amounts in all seeds. Some seeds, especially 



