132 



MORPHOLOGY, OE COMPARATIVE ANATOMY. 



with only a single cavity ; and as the lines of junction of the car- 

 pels are on the outer wall, the placentas must stand inside those 

 lines ; in this way is formed a unilocular compound ovary with 

 parietal placentas (figs. 253-255). There are no dissepiments : 

 and the ventral sutures, alternating on the outer walls with the 

 dorsal sutures, are, in such cases, like the placentas within, formed 

 of the confluent margins of two different carpels instead of those 

 of the same carpel. 



We find almost every possible degree of transition between the parietal 

 and the axile placentas, according as the placentiferous margins project 

 more or less into the interior of the ovary. True parietal placentas are 

 found in Violaceae (fig. 254), Gentianacese (fig. 253), Cistaceae (fig. 255), 

 &c. In Papaver we have the margins turned-in so as nearly to reach the 



Fig. 252. 



Fig. 253. Fig. 254. Fig. 255. 



Fig. 252. a, legume of Asfragalus ; 6, cross 

 section, showing a false dissepiment formed 

 by the inflexion of the suture. 



Unilocular compound ovaries. 



Fig. 253. Ovary of Gentianaceae. 

 Fig 254. Ovary of Viola. 

 Fig. 255. Ovary of Cistus. 



centre (as imperfect dissepiments) ; in some Hypericacese (If. araveolens) 

 the originally axile placentas become parietal by separation during the 

 ripening of the fruit, while in Cucurbitacese the originally distinctly 

 parietal, although greatly infiexed, margins ultimately cohere so as to form 

 an axile placenta. 



In Cruciferse we have an anomalous condition, where there are two 

 double parietal placentas, but from the central line of each projects a plate 

 passing across the cavity and forming 'a kind of spurious septum, called 

 a replum ; so that each cell contains only the two half-placentas formed 

 by its own margins. 



Free central Placentas. In some Orders, where the walls are 

 as in the unilocular compound ovaries above described, the placentas 

 are found as a free column or expanded mass in the centre of the 

 common cavity. This forms the compound unilocular ovary with a 

 free central placenta. In Primulacese, Santalacea), and some other 

 Orders, where this kind of placentation occurs, the placentas are 

 free from their very earliest state, and are seen to be direct pro- 

 longations of the receptacle or axis within the carpels. 



