286 ORGANOGRAPHY. 



from the margins of the carpels in axile and parietal placcnta- 

 tion, may be considered, tliereforc, as capable of being proved 

 by direct obsen'ation, and from analogy to wliat occurs in 

 certain ordinary leaA^es. 



We now pass to consider the origin of the free central pla- 

 centa. The theory formei-ly entertained was, that this also 

 was a development from the margins of the carpels. It was 

 thought that the carpels of which the compound ovary was 

 formed, originally met in the centre and developed placentas 

 from their margins in the same manner as in ordinary axile 

 placentation, but that subsequently the Avails of the ovary 

 grew more rapidly than the dissepiments, so that the connexion 

 between them was soon destroyed ; and that from this cause, 

 and also from the great subsequent development of the placenta, 

 the septa ultimately became almost or quite broken up, so that 

 it was left free in the cavity of the ovary. This tlieory is 

 strengthened by the fact, that in several of the Caryophyllere, we 

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

 {fig. 617), whence it may be concluded, that these are the re- 

 mains of dissepiments Avhich become ruptured on account of the 

 unequal development of the parts of the ovary. In the Prim- 

 rose, however, and many other plants, Avhich 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 Primulacea;, 

 state, that the placenta is free in the centre from its earliest ap- 

 pearance; that it is originally a little papilla on the apex of the 

 thalamus, and that the walls of the future ovary grow up per- 

 fectly free, and ultimately enclose it. The formation of such a 

 free central placenta cannot be well explained upon the mar- 

 ginal theory, as the carpels have never had any connexion with 

 it except at their base. Hence this kind of placentation has 

 been supposed by Schlciden, Endlicher, and many other bota- 

 nists, 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 Primula, 

 and hence such a placenta has been denominated axile by some 

 botanists ; but as this name had been ])reviously applied to 

 another form, the adoption of such a term cannot but lead to 

 much confusion. The free central placenta of Primula can only 

 be explained on tlie marginal or car])ellary theory of the form- 

 ation of ]jlacentas, by supposing, either that the placentas arc 

 only i)roduccd at the base of tlie carpels, ar.d subsecjucntly 

 elongate and enlarge, or that they are formed by a wliorl of 

 j)laceiitas developed sejjarately from the car])els by a j.rocess of 

 chorisis, and that these afterwards become united in tlie centre 

 of the ovary. 



