MOBPHOLOGY OF REPRODUCTIVE ORGANS 199 



The term placenta is commonly applied to the more or 

 less marked projection occurring in the cavity of the ovary 

 (figs. 398, p, and 431, pi), to which the ovule or ovules 

 are attached. The placentas are variously distiibuted in the 

 ovaries of different plants, but their arrangement is always the 

 same for those of any particular species, and is frequently con- 

 stant throughout entire genera, or even natural orders ; hence 

 accurate discrimination of them is of great practical importance. 

 The term placentation is used to indicate the manner in which 

 the placentas are distributed. 



a. Kinds of Placentation. — In the monocarpellary ovary the 

 placenta is usually situated at the ventral suture or the line 

 which corresponds to the union of the two margins of the 



Fig. 432. 



Fig. 433. 



Fig. 434. 



Fig. 432. Transverse section of the compound ovary of the Lily. The ovary 



is three-celled (trilocular). The placentas, pi, are axile or central. 



Fig. 433. Transverse section of the ovary of a species of Campanula. The 

 ovary is five-celled or quinquelocular, and the placentation, pi, axile. 

 Fig. 434. Transverse section of the ovary of a species of Cactus. Ttie 

 ovary is one-celled and the placentation parietal. 



blade of the carpellary leaf (figs. 398, 399, and 431), out of 

 which it is formed ; such a placenta is therefore usually termed 

 marginal. 



In compound ovaries we have three kinds of placentation ; 

 namely, axile, parietal, and free central. The axile form occurs 

 generally in compound many-celled ovaries, because in these 

 each of the ovaries of the component carpels is placed in a 

 similar position to that of the simple ovary (figs. 415 and 416), 

 and hence the placentas situated at their ventral sutures are 

 arranged in the centre or axis, as in the Lily (fig. 432) and 

 Campanula (fig. 433). 



In a compound one-celled ovary there are two forms of 

 placentation ; namely, the parietal and the free central. The 

 placentation is tenaed parietal when the ovules are attached 



