. 
ORIGIN OF THE PLACENTA. 285 
young ovary (/ig. 632) ; and even traces of these at the lower 
part of the mature ovary ; hence it may be concluded that these 
are the remains of dissepiments which have become ruptured 
on account of the unequal development of the parts of the 
ovary. In the Primrose, however, and many other plants, 
which have a free central placenta, no traces of dissepiments 
ean be found at any period of the growth of the ovary. 
Duchartre, and others also, who haye traced the development 
of the ovary in the Primulacex, state that the placenta is free 
in the centre from its earliest appearance ; that it is originally 
a little papilla on the apex of the thalamus, and that the walls 
of the future ovary grow up perfectly free, and ultimately 
enclose it. The formation of such a free central placenta can- 
not therefore be well explained upon the marginal theory, as 
the carpels have never had any connexion with it except at 
their bases. Hence this kind of placentation has been supposed 
by many botanists not to be formed from the carpels at all, but 
to be a prolongation of the axis, which bears ovules, instead of 
buds as is the case with branches. This theory explains very 
readily the formation of the free central placenta of Primula, 
and hence such a placenta has been denominated axile by some 
botanists ; but this name, as already noticed (page 282), having 
been already applied to another kind of placentation, the adop- 
tion of such a term cannot but lead to much confusion. The 
free central placenta of Frimula can only be explained on the 
marginal or carpellary theory of the formation of placentas, by 
supposing, either that the placentas are only produced at the 
base of the carpels, and subsequently elongate and enlarge ; or 
that they are formed by a whorl of placentas developed sepa- 
rately from the carpels by a process of chorisis (see Chorisis), and 
that these afterwards become united in the centre of the ovary. 
Schleiden, indeed, and some other botanists regard the 
placenta in all cases as a development from the axis of the 
plant. The axile and free central placentation are readily to be 
explained by it, but the formation of the parietal placenta is by 
no means so clear. It is supposed in the latter case that the 
axis ramifies in the cavity of the ovary, and that the branches 
curve directly from their origin towards the side, and become 
blended with the margins of the two adjoining carpels on their 
inner side, and form parietal placentas bearing ovules as lateral 
buds. Schleiden thinks that the formation of the ovule in the 
Yew, where it terminates a branch, and is naked, is incom- 
patible with the marginal theory. He also believes that the 
formation of the ovules generally in the Coniferze supports his 
_ views of placentation. He regards the ovules in these plants 
as being given off from the axis of the cone, which he calls 
a placenta, and the scales, or bracts, which are situated between 
them, he maintains are open carpellary leaves. Schleiden also 
states, that no satisfactory explanation can be given by the 
