STRUCTURE OF THE OVULE. 329 
being commonly situated at the opposite end of the sac to the 
germinal vesicles, that is, at its base. These cells have a cell- 
wall formed of cellulose ; but their purport is unknown, and 
their existence is temporary, as they disappear after fertilisation. 
Some ovules, as those of the Mistletoe ( jig. 739), consist simply 
of the nucellus, 1, and embryo-sac, c,as above described, in which 
case the nucellus is termed waked (fig. 738) ; but in almost all 
plants it becomes enclosed in one or, two coats. Thus, in the 
Walnut there is but one coat, which appears at first as a little 
circular process around its base ; this gradually increases in size, 
and by growing upwards ultimately forms a sheath or cellular 
coat to the nucellus, which it entirely closes except at the apex, 
where a small opening may be always observed (fig. 740, end). 
The coat thus formed, where there is but one, is called the in- 
tegumentum simplex, s; and the orifice, end, at the apex of the 
nucellus, n, is termed the micropyle or foramen. Besides the 
HIG. Vos. Fie. 739. Fie. 740. Fie. 741. 
Fig. 738. Ovule of the Mistletoe (Viscum album), consisting of a naked 
nucellus.— Fig. 739. The same ovule cut vertically to show the embryo- 
sac, c, in the nucellus, n. Fig.740. Ovule of the Walnut (Juglans regia). 
n. Projecting end of the nucellus. s. Coat covering the nucellus except 
at the foramen, end.— Fig. 741. Ovule of a species of Polygonum. 7. End 
of ovyule where it is attached to the placenta. p. Primine. s, Secundine, 
ex. Exostome, end. Endostome. x. Projecting end of the nucellus. 
Walnut, there is only one coat formed in the Compositze, Cam- 
panulaceze, Lobeliacezx, and some other orders. 
In most plants, however, the ovule has two coats, in which 
case we observe two circular or annular processes around the 
base of the nucellus, the inner one being first developed ; these 
processes continue to grow upwards as before described, until 
they also ultimately form two sheaths or coats, which entirely 
enclose the nucellus except at its apex (jig. 741). The inner 
coat is at first seen to project beyond the outer, but the latter 
ultimately reaches and’ encloses it. The inner coat is usually 
termed the secundine ( figs. 741, s, and 742, c), and the outer the 
primine ; but some botanists, following the order of development 
of the coats, term the inner coat the primine, and the outer the 
secundine, thus reversing the order of names as above mentioned. 
Others, to prevent confusion, more properly term the inner coat 
or secundine, the integwmentwm internwm ; and the outer coat, 
or primine, the integumentum externum. The orifice left at 
