FORMATION OP THE PLACENTA. 



215 



441. The carpellary 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 thicken- 

 ings along the walls. In fig. 390, the pistil of Viola tricolor is repre- 

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

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

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

 placentas 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 Caryo- 



392 



phyllacese. Thus, in Cerastium hirsutum (figs. 391, 392), the ovary, 

 o, is composed 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, <?, attached to it. 



442. 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 Primulaceae, Myrsinaceae, Santalaceas, and Theo- 

 phrasteas, in which no vestiges of septa or marginal ovules can be 

 perceived at any period of growth. Duchartre states that the free 

 placenta of Primulacese, is totally different from that of Caryophyl- 

 laceas. 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, according to him, wholly through- 

 Pig. 390. Pistil of Viola tricolor, or Pansy, cut vertically to show the ovules, o, attached to the 

 parietes. Two rows of ovules are seen, one in front, and the other in profile, p, A thickened 

 line on the walls forming the placenta, c, Calyx, d, Ovary. 1. Hooded stigma terminating the 

 short style. 2. Horizontal section of the same, p, Placenta, o, Ovules, s, Suture. 



Fig. 391. Pistil of Cerastium hirsutum cut vertically, o, Unilocular or monothecal ovary, p, 

 Free central placenta, g, Ovules, s, Styles. 



Fig. 392. 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. 



