FOEMATION OF THE PLACENTA. 241 



is hence sometimes called marginal. The placenta is formed on each 

 margin of the carpel, and hence is essentially double. This is seen 

 in cases where the margins of the carpel do not nnite, but remain 

 separate, and consequently two placentas are formed in place of one. 

 In fig. 420 the two carpels are folded, so that their margins meet, 

 and the placenta is apparently single ; whereas in fig. 421 the margins 

 of each carpel do not meet, and the placenta of each is double. 

 Again, in fig. 422, the two carpels, after meeting in the centre or axis, 

 a, are reflected outwards towards the dorsal suture, sd, and their margins 

 separate slightly, each being placentiferous, and bearing ovules, o. 



When the pistil is formed by one carpel, the inner margins unite 

 in the axis, and form usually a common marginal 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 

 (dissepio, 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 laminse ; 

 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. 



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, guinquelocular, or multilocular, 

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

 each corresponding to a single cell or loculament (fig. 415, 2, ce, ci). 

 In these cases the marginal placentas meet in the axis, and unite so 

 as to form a single central one (fig. 420 a). The number of locula- 

 ments is equal to that of the dissepiments. In fig. 418 there is 

 shown a transverse section of the ovary of Fuchsia coocinea, c c c c 

 being its parietes formed by the union of four carpellary leaves, a th« 

 axis united to the parietes by dissepiments, and o the ovules attached 



Kgs. 420, 421, 422. Horizontal sections of ovaries, oomposed of two carpellary leaves, 'the 

 edges of whicli are folded so as to meet in the axis, a, in fig. 420 ; are turned inwards into 

 the loculaments after meeting in the axis in fig. 422 ; and do not reach the axis in fig. 421. 



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