160 



starch, and oil are highly valuable for human food, and so is 

 sugar, which occurs in small quantities in all the bread grains 

 and in considerable amounts in the best varieties of sweet corn. 



The selection and cultivation of plants 

 like the grains, which contain much digest 

 ible food in a concentrated form, and which 

 may retain their food value for some years 

 with little loss, marked a long step upward 

 in the civilization of the human race. 



151. The seed coat. The seed coat is 

 more or less efficient in protecting its con- 

 tents from mechan- 

 ical injury, such as 

 crushing, and in 

 many cases it pro- 

 tects the more per- 

 ishable materials 

 within it from de- 

 cay. Before germi- 

 nation can begin, 

 a certain amount 

 of moisture must 

 usually soak into 

 the seed, either 

 through the gen- 

 eral surface, as in 

 most seeds, or, in 

 such hard-shelled 

 seeds as the coco- 

 nut, hickory nut, 

 walnut, and but- 

 ternut, through a thin or soft place in the wall. Usually the 

 little opening in the ovule, known as the micropyle (fig. 119, m), 

 remains in the seed and serves to admit moisture. 



The coats of many seeds have wings or outgrowths of hairs 

 which aid in their dispersal. Other modifications in the coats 



FIG. 143. Grain and seedling of corn 



A, lengthwise section of grain; B, the embryo re- 

 moved; C, seedling; en, endosperm ; em, embryo; r.s, 

 sheath covering tip of rudimentary root ; sc, scutellum, 

 or absorbing cotyledon ; pi, plumule ; sh, sheath-like 

 leaf in which the first foliage leaves are inclosed ; r, 

 first root, springing from within r.s ; r' later-formed 

 roots arising from other parts of the grain 



