ae 
oF 
286 ORIGIN OF THE PLACENTA. 
advocates of the marginal theory of placentation of the forma- 
tion of the ovule and placenta in Armeria, in which the ovary 
composed of five carpels surrounds a single ovule, which rises 
from the bottom of the axis, supported on a stalk which curves 
downwards at its apex, and thus sus- 
Fie. 637. pends the ovule free in the centre of 
the cavity (fig. 637). He accordingly 
concludes, that the ovule and placenta 
are developments of the axis. Many 
other arguments in favour of the uni- 
versal applicability of the axial theory 
in the formation of the placenta have 
been brought forward by Schleiden 
and other botanists, but their further 
discussion would be out of place here. 
From all that has now been stated, 
we may draw the following conclusions, 
namely :—that no one theory suffi- 
ciently accounts for the production of 
mi i ene prac aes the ms Psa in all cases oe that the 
rey : mie a“ contain euly axile and some forms of the free central 
from » funioulas ot atalk, Placentation may be explained on beth 
The ovule is here said to be hypotheses ; that the parietal placenta- 
es tion is best explained upon the marginal 
theory ; and that the formation of the 
free central placenta of the Primulace, Santalacez, and some 
other plants, can only be satisfactorily explained by considering 
the placenta as a production of the axis. 
In a practical point of view, the mode of production of the 
placenta is of little importance. The accurate discrimination of 
the different kinds is, however, of much value in Descriptive 
Botany, by affording us constant, and hence important characters 
for distinguishing plants. Some natural orders exhibit more 
than one variety of placentation, and cannot be therefore distin- 
guished by any particular kind ; hence, in such orders, the pla- 
centation can only be applied in obtaining good characteristics 
of the genera. In the majority of instances, however, we find 
one kind of placentation occurring throughout all the plants of a 
particular natural order. Thus, the Scrophulariacez, Ericacez, 
and Campanulaceze present us with axile placentation ; the 
Papaveraceze, Violaceze, and Cruciferze with parietal; and the 
Caryophyllacere, Santalaceze, and Primulaceze, with free central 
placentation. 
2. Tue Styte.—We have already described (page 270) the 
general nature and structure of the style in speaking of the 
carpel. There are, however, certain other matters connected 
with it still to be alluded to. 
The style usually arises from the geometrical summit of 
the ovary, of which it is a continuation in an upward direction, 
