214 FORMATION OP THE PLACENTA. 



This placenta may extend along the whole margin of the ovary as far 

 as the base of the style, or it may be confined to the base or apex 

 only. When the pistil is composed of several separate carpels, or, in 

 other words, is apocarpous, there are generally separate placentas at 

 each of their margins. In a syncarpous pistil, on the other hand, the 

 carpels are so united that the edges of each of the contiguous ones by 

 their union form a septum (septum, a fence or enclosure), or dissepiment, 

 (ctissepio, I separate), and the number of these septa consequently in- 

 dicates the number of carpels in the compound pistil. It is obvious 

 then that each dissepiment is formed by a double wall or two laminae; 

 that the presence of a septum implies the presence of more than one 

 carpel; and that, when carpels are placed side by side, true dissepi- 

 ments must be vertical, and not horizontal. 



440. When the dissepiments extend to the centre or axis, the 

 ovary is divided into cavities, cells or loculaments (loculus, a box), and 

 it may be bilocular, trilocular, quadrilocular, quinquelocular, or multi- 

 locular, according as it is formed by two, three, four, five, or many 

 carpels, each corresponding to a single cell or loculament (fig. 381, 2, 

 c e, c i). In these cases the marginal placentas meet in the axis, and 

 unite so as to form a single central one (fig. 386 a). Some call this 

 placentation axile (belonging to the axis), but this term is perhaps pro- 

 perly restricted to cases where the placenta is an actual prolongation 

 of the axis. The number of loculaments is equal to that of the dis- 

 sepiments. In fig. 384, there is shown a transverse section of the ovary 

 of Fuchsia coccinea, c c c c being its parietes formed by the union of 

 four carpellary leaves, a the axis united to the parietes by dissepiments, 

 and o the ovules attached to the placentas at the margin of each carpel. 

 When the carpels in a syncarpous pistil do not fold inwards completely 

 so as to meet in the centre, but only partially, so that 

 the dissepiments appear as projections on the walls 

 of the ovary, then the ovary is unilocular (fig. 387), 

 and the placentas are parietal (paries, a wall). A 

 horizontal section of the ovary of ErythraeaCentaurium 

 '7> (fig- 389), exhibits a unilocular ovary with parietal 

 placentas, p, formed at each of the margins of the 

 carpels which do not meet in the centre. In these 

 instances the placentas may be formed at the margin 

 of the united contiguous leaves, so as to appear single, 

 or the margins may not be united, each developing a placenta. From 

 this it will be seen that dissepiments are opposite to placentas, formed 

 by the union of the margins of two contiguous carpels, but alternate 

 with those formed by the margins of the same carpel. 



Fig. 389. Horizontal section of the ovary of Erythraa Centaurinm. c, Wall or paries of the 

 ovary or carpellary leaf, p, The edge on which the placenta is formed, bearing the ovules, o. 

 I, Cell or loculament. 



