ORGANS OF REPRODUCTION. 279 



placenta was left free in the cavity of the ovary. This theory is 

 strengthened by the fact, that in several of the Caryophyllacese, 

 we often find traces of dissepiments at the lower part of the 

 ovary (z?^. 618), whence 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 can be found at any 

 period of the growth of the ovary. Duchartre, and others also, 

 who have traced the development of the ovary in the Primu- 

 lacese, state, that the placenta is free in the centre from its 

 eai'liest 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 forma- 

 tion of such a free central placenta cannot be well explained 

 upon the marginal theory, as the carpels have never had any 

 connexion with it except at their base. Hence this kind of 

 placentation has been supposed by Schleiden, Eudlicher, and 

 many other 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 generally. This theory 

 explains very readily the formation of the free central placenta 

 of Pri/Hul-a, and hence such a placenta has been denominated 

 axile by some botanists ; but this name having been already 

 applied to another form of placentation, the adoption of such a 

 term cannot but lead to much confusion. The free central pla- 

 centa of Primula can only be explained on the marginal or car- 

 pellary 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 separately from the carpels 

 by a process of chorisis, and that these afterwards become united 

 in the centre of the ovary. 



Schleiden, indeed, and some other botanists regard the pla- 

 centa in all cases as a development from the axis of the plant. 

 Schleiden, particularly, has written an elaborate paper in support 

 of this view. 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 incompatible with 

 the marginal theory. He thinks, also, that the formation of the 

 ovules generally in the Coniferse, supports his views of placen- 

 tation. He regards the ovid.es in these plants as being given off 



