Colorado Weed Seeds 
15 
wheat, (Fig. 2), whe.'it, 
corn, and other cereals, 
there is but one ovule. 
Thus we see that the true 
seed, botanically speak- 
ing, is always associated 
with the flower structure, 
and is formed within the 
ovary of the pistil ; the 
seed develops from a 
structure in the ovary 
known as the ovule. In 
ized and ripened ovule. 
It is primarily a young plant in a resting stage capable of develop- 
ment by germination. 
Seed development begins immediately after the union of a 
male element of the pollen tube with the egg or female element 
of the ovule. The pollen tube enters the ovule by way of the 
micropyle. The body resulting from this union of two sexual 
elements develops into the young plant or embryo. The embryo 
may be partly or wholly imbedded in the endosperm which is a 
food supply, as in wheat and other cereals, or may occupy the 
entire space within the seed coats when the endosperm is lacking. 
The latter condition is characteristic of bean and pumpkin seeds, 
and is due to the absorption of the endosperm by the embryo dur- 
ing the development of the seed. The cells of the nucellus are in 
part absorbed by the developing embroyo and the nucellus is 
usually present as a thin, compressed layer known as perisperm. 
During the process of development of the embryo and nucellus, 
changes have taken place in the coats of the ovules. 1 he 
integuments, or outside layers, have gradually hardened and 
formed seed-coats. 
In describing the kinds of ovules the terms atropous (orthotro- 
pous), campylotropous, amphitropous, and anatropous are used 
(Fig. 3). The same terms may be used in describing seeds, par- 
ticularly in referring to the relative positions of hilum and 
micropyle. -The seed upon ripening usually detaches itself from 
the stalk (funicle) and the resulting scar is called the hilum. The 
