160 INTRODUCTION TO BOTANY 
starch, and oil are highly valuable for human food, and so is 
sugar, which oecurs 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 
Fre, 143. Grain and seedling of corn 
: -l, lengthwise section of grain; B, the embryo re- 
most seeds, or, IN moved; C; seedling; en, endosperm; em, embryo; r.s, 
such hard-shelled sheath covering tip of rudimentary root; sc, scutellum, 
or absorbing cotyledon; pl, plumule; sh, sheath-like 
seeds as the coco- leaf in which the first foliage leaves are inclosed; r, 
nut, hickory nut, first root, springing from within 7.s; 7” later-formed 
roots arising from other parts of the grain 
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, av), 
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 
