PARIETAL PLACENTATION. 



303 



are here borne upon the extremity of the imperfect dissepiments, 

 which, if somewhat 

 prolonged, would 

 meet and unite in 

 the centre, so as to 

 present the regular 

 three-celled struc- 

 ture (as in Fig. 



383). This will be evident on comparing the pod of the Common 

 St. John's-wort (figured under Ord. Hypericacese), which is com- 

 pletely three-celled with the placenta? united in the axis, with the 

 ovary of another species (Fig. 388), where the 

 three placenta touch in the centre without co- 

 hering, and with the full grown pod of the last 

 (Fig. 389), where they are drawn asunder by 

 the expansion of the growing pod, and remain 

 attached only to its walls, borne on three slight 

 introflexions, which stand in the place of dissepi- 

 ments. Parnassia affords a similar instance, 

 only there are usually four such placentae in- 

 stead of three (Fig. 304, the centre of which 

 represents a cross-section of the 4-carpellary 

 ovary). These instances bring us to the fre- 

 quent case in which we may say that the leaves 

 of the gynsecial verticil, placed merely in apposition, as in valvate 

 aestivation (499), directly cohere into one circle by their respective 

 contiguous margins ; which, being barely induplicate, form pla- 

 centse which are borne directly on the walls. This is shown in 

 the diagram, Fig. 386, representing a cross-section of three carpels 

 thus combined into a compound one-celled ovary, without any ap- 

 pearance of dissepiments. Thus borne upon the walls, instead of 

 in the axis, of the compound ovary, the placentse are said to be 

 parietal. Examples of the kind with a tricarpellary ovary are 

 furnished by many Hypericums, by the Violet Family, the Cistus 



FIG. 385 - 387. Diagrams illustrating parietal and free central placentation. 385. Cross- 

 section of an ovary composed of three united carpels, where the introflexed portions do not 

 reach the centre. 386. Section of a similar ovary, except that the placental margins unite 

 without any introflexion (placentae strictly parietal). 387. Section of a tricarpellary ovary, 

 with a free central placenta, produced by the obliteration of the dissepiments. 



FIG. 388. Magnified cross-section of the ovary of Hypericum graveolena. 389. Enlarged 

 cross-section of the mature pod of the same, where the placentae become strictly parietal. 



