FORMATION OF THE PLACENTA. 243 



are not connected with the walls of the ovary, and form what is called 

 a free central placenta. This is seen in many of the Oaryophyllacese. 

 Thus, in Cerastium hirsutum (figs. 425, 426), the ovary, o, is com- 

 posed of five carpels, indicated by the styles, s, but there is only one 

 loculament, the placenta, p, being free in the centre, and the ovules, 

 g, attached to it. 



In Caryophyllacese, however, while the placenta is free in the 

 centre, there are often traces found at the base of the ovary of the 

 remains of septa, as if rupture had taken place ; and, in rare instances, 

 ovules are found on the margins. But examples occur of this kind 

 of placentation, as in Primulacese, Myrsinacese, and Santalacese, in 

 which no vestiges of septa or marginal ovules can be perceived at any 

 period of growth. The free placenta of Primulacese is different from 

 that of Oaryophyllacese. It is always free, and rises in the centre of 

 the ovary, and the part uncovered by ovules gradually extends into the 

 style. It is not first continuous with the style, and then free ; neither 

 is it originally marginal and then free ; but it is, throughout its 

 organogeny (ogyavov, organ, and y'svigi;, production or development), 

 separate and axile. 



Free central placentation, therefore, has been accounted for in two 

 ways : either by supposing that the placentas in the early state were 

 formed on the margins of carpellary leaves, and that in the progress 

 of development these leaves separated from them, leaving the placentas 

 and ovules free in the centre ; or by supposing that the placentas are 

 not marginal but axile formations, produced by an elongation of the 

 axis, the ovules being lateral buds, and the carpels verticillate leaves, 

 united together around the axis. The latter view has been supported 

 by many botanists, and is confirmed by the fact that in some cases 

 the placenta is actually prolonged beyond the carpels. The first of 

 these views would apply well to Oaryophyl- 

 laceae, the second to Primulacese. The latter 

 case has been explained, on the marginal 

 hypothesis, by considering the placentas as 

 formed from the carpels by a process of 

 chorisis, and united together in the centre. 



Some consider the axile view of placenta- 

 tion as applicable to all cases, the axis in some 

 cases remaining free and independent, at 

 other times sending prolongations along the 

 margins of the carpellary leaves, and thus 

 forming the marginal placentas. The oc- ^'^- *^'- 

 currence of placentas over the whole inner surface of the carpels or of 



Figs. 427, 428. One of the carpels of Butonius mabellatus, or flowering Eush, cut trans- 

 versely in 427, and longitudinally in 428. I, Loculament or cavity of the carpel, o, Ovules. 

 s, Stigmata. 



