330 RELATION OF HILUM, CHALAZA, AND MICROPYLE. 
the apex of the nucellus, as in the former instance where only 
one coat is present, is called the foramen or micropyle. The 
Fie 0729; openings in the two coats commonly cor- 
respond to each other, but it is sometimes 
found convenient to distinguish them by 
distinct names ; thus, that of the outer is 
called the exostome (fig. 741, ex); that of 
the inner, endostome, end. In some plants, 
as in Welwitschia, the primine appears as 
a prolonged tubular body beyond the apex 
of the ovule, in which case it closely re- 
Dd sembles a style. 
/ The nucellus and its coat or coats are 
Fig. 742. Section of an intimately connected at one point by a 
ovule (diagrammatic). cellulo-vascular cord or layer, called the 
om eee ote aE chaiuza (figs. 743, ch, and 744, ch); but at 
Outer coat. e, Micro- the other parts of the ovule they are more 
Bee es ee g- ov less distinct. This chalaza is the point 
i ‘where the vessels pass from the placenta, 
or when the ovule is stalked from the funiculus, into the ovule, 
for the purpose of affording nourishment to it ; it is generally 
indicated by being coloured, and of a denser texture than the 
tissue by which it is surrounded. The chalaza is by some con- 
sidered as the organic base of the ovule, and the micropyle as 
the organic apex ; but it is better to speak of the hilum as the 
organic base of the ovule, and the chalaza as the base of the 
nucellus. Through the micropyle the influence of the pollen 
is conveyed to the embryo-sac, as will be hereafter fully 
described. 
The development and structure of the ovules as described 
above refer only to those of the Angiospermia ; those of the 
Gymnospermia present some very striking differences, which 
will be described hereafter, when speaking of their repro- 
duction. 
RELATION OF THE Hitum, CHALAZA, AND MICROPYLE TO ONE 
ANOTHER. — When an ovule is first developed, the point of union 
of its coats and nucellus, called the chalaza, is at the base or 
hilum, close to the placenta or funiculus; in which case a 
straight line would pass from the micropyle through the axis of 
the nucellus and its coats to the hilum. In rare instances this 
relation of parts is preserved throughout its development, as 
in the Polygonaceze (fig. 743) ; when the ovule is termed ortho- 
tropous, atropous, or straight. In such an ovule, therefore, the 
micropyle, m, would be situated at its geometrical apex, or at 
the end farthest removed from the hilum, in which case the 
organic apex would correspond to the geometrical apex ; while 
the chalaza, ch, would be placed at the base of the ovule or 
hilum. 
It generally happens, however, that the ovule, instead of 
