DEVELOPMENT OF THE OVUWLE. 327 
In the above cases the position of the ovule is in general 
constant, and hence this character is frequently of much impor- 
tance in distinguishing genera, and even natural orders. Thus, 
in the Compositze the solitary ovule is always erect ; while in the 
allied orders, the Valerianaceze and Dipsacacez, it is suspended 
or pendulous ;—the two latter terms are frequently used in- 
differently by botanists. In the Polygonacez (jig. 731), the 
ovule is also always solitary and erect ; and in the Thymelace 
(fig. 734), itis suspended. In other natural orders we find the 
position varying in different genera, although generally constant 
in the same ; thus, in the Rosacez, the genera Gewm, Alche- 
mula, and others, have an ascending ovule, while those of 
Poterium, Sanguisorba, &c., have it suspended, and in Potentilla 
both ascending and suspended ovules are found. In the 
Ranunculacez also we find the ovule varying in like manner as 
regards its position. 
We will now consider the position of the ovules when their 
number is more than one. Thus when the ovary or cell has two 
ovules, these may be either placed side by side at the same level 
and have the same direction, as in Nuttallia, when they are said 
to be collateral ; or they may be placed at different heights, and 
then they may either follow the same direction, when they are 
superposed, or one ovule may be ascending and the other sus- 
pended, as in Asculus (fig. 735). The position of the ovules in 
those cases where they are in definite numbers, is also usually 
constant and regular, and similar terms are employed ; but 
when the number of ovules in the ovary or cell is indefinite, the 
relations are less constant, and depend in a great measure upon 
the shape of the cell and the size of the placentas. Thus in the 
long ovaries of many of the Leguminosz and Crucifere (jig. 615), 
the ovules are superposed, and by not crowding each other they 
will all be turned in the same direction ; while, on the contrary, 
if the ovules are numerous, and developed in a small space, they 
will necessarily press against each other, and acquire irregular 
forms and varying positions, according to the direction of the 
pressure. In describing these varying positions the same terms 
are used as those referred to when speaking of the relations of 
the solitary ovule. These terms are also applied in the same 
sense to the relations of the seed in the pericarp. 
DEVELOPMENT AND STRUCTURE OF THE OvuLE.—The ovule 
first appears on the placenta as a little conical cellular projection, 
which gradually enlarges and ultimately acquires a more or less 
rcunded or oval form, which is sessile or stalked, andistermed the 
nucellus or wucleus (fig. 738), and which may be regarded as corre- 
sponding to the megasporangium of some of the vascular Crypto- 
gams (page 369). This nucellus is at first perfectly uniform in 
texture and appearance. presenting no cavities except those of 
the ordinary parenchymatous cells of which it is composed, and 
having no integuments or coats ; but as development proceeds 
