242 



FORMATION OF THE PLACENTA. 



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. 421), and the placentas are parietal {'paries, a 

 wall). A horizontal section of the ovary of Ery thrsea 

 Oentaurium (fig. 423) exhibits a unilocular ovary 

 with parietal placentas, p, formed at the inner 

 margins of each 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. Prom 

 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. 



The carpeUary leaves may fold inwards very slightly, or they may 

 be applied in a valvate manner, merely touching at their margins, the 

 placentas then being parietal, and appearing as lines or thickenings 

 along the walls. In fig. 424 the pistil of Viola tricolor is represented, 

 1, cut vertically, and, 2, cut transversely, the ovules being attached 



this it will be 



Fig. 424. Fig. 426. Fig. 426. 



to the walls of the ovary, and the placentas, p, being merely thickened 

 portions of the walls. Cases occur, however, in which the placentas 



Fig. 423. Horizontal section of the ovary of Erythrsea Centanrium. c. Wall or paries of 

 the ovaryDr carpeUary leaf, jj. The edge on which the placenta is formed, hearing the 

 ovules, 0. I, Cell or loculament. Fig. 424. Pistil of Viola tricolor, or Pansy. 1, Vertical 

 section to show the ovules, u, attached to the parietes. Two rows of ovules are seen, one 

 in front, and the other in profile, jj, A thickened line on the walls forming the placenta, 

 c. Calyx, d. Ovary, s. Hooded stigma terminating the short style. 2, Horizontal section 

 of the same. j3. Placenta, o. Ovules, s. Suture, Fig, 425. Pistil of Cerastium hirsutum 

 cut vertically, o. Unilocular or monothecal ovary, jj. Free central placenta, g, Ovules, 

 s. Styles. -Fig. 426. The same cut horizontally, and the halves separated so as to show the 

 interior of the cavity of the ovary o, with the free central placenta, p, covered with 

 ovules, g. 



