138 



PRACTICAL BOTANY 



man and other animals generally consists either of seeds them- 

 selves, as in the case of the grains, nuts, beans, and peas, or 

 of manufactured products, such as oatmeal, corn meal, flour, 

 cornstarch, cottonseed oil, derived from seeds. 



The principal plant foods found in the seed are proteins 

 of many kinds ; carbohydrates in the form of starch, sugar, or 

 cellulose ; and fats or oils. The characteristics of these vari- 

 ous substances can be learned only by means of careful labo- 

 ratory work, though some of them are tolerably familiar to 

 most people. Not infrequently the different kinds of reserve 

 material are localized in special parts of the 

 seed. In the grain of wheat and of corn the 

 proteins .are especially abundant in the trans- 

 lucent flinty outer part of the endosperm, 

 while the starch lies mainly in the interior 

 white portion (Fig. 333). The oil of the corn 

 grain is stored mainly in the embryo, so that 

 kinds which have large embryos contain a 

 high percentage of oil and those with small 

 embryos have a low percentage (Fig. 334). 



Every seed must contain some protein 

 material, since this is indispensable to the 

 building of protoplasm, and no growth can 

 take place without it. But it does not seem 

 to make much difference whether the non-nitrogenous food in 

 the seed consists mainly of starch as in rice, of oil as in Brazil 

 nuts, or of cellulose as in coffee and date seeds. Along with 

 much starch, many of the grains, particularly millet, contain 

 a good deal of gum, sugar, and fat. The fact that sugar is not 

 usually abundant in seeds may be due to the readiness with 

 which it dissolves in water, which might lead to some of it 

 becoming lost in the soil during germination. 



129. The seed coat. The seed coat protects the embryo (and 

 the endosperm, when present) from mechanical injuries. In 

 order to allow germination to begin, either the general surface 

 of the coat must, as in most seeds, be porous enough to absorb 



FIG. 128. Diagram 

 of lengthwise sec- 

 tion of a grain of 

 wheat 



en, endosperm ; em, 



embryo. Somewhat 



magnified 



