108 



COMPOUND PISTILS. 



[section 10. 



Such a pistil is just what would be formed if the simple pistils (two, three, 



or five ill a circle, as the case may be), lilte those of a Paeony or Stonecrop 



(Fig. 224, 225), pressed together in the centre of the flower, 



were to cohere by their contiguous parts. In such a case 



the placentae are naturally axile, or all brought together in 



the axis or centre ; and the ovary has as many Dissepiments, 



or internal Partitions, as there are carpels in its composition. 



Eor these are the contiguous and coalesceut walls or sides of 



the component carpels. When such pistils ripen into pods, 



they often separate along these lines into their elementary 



carpels. 



311. One-celled, with free Central Placenta. The 

 commoner case is that of Purslane (Fig. 272) and of the 

 Pink and Chickweed families (Pig. 331, 332). This is ex- 

 plained by supposing that the partitions (such as those of 

 Fig. 329) have early vanished or have been suppressed. In- 

 deed, traces of them may often be detected in Pinks. On the other hand, 

 it is equally supposable that in the Primula family the free central is de- 

 rived from parietal placentation by the carpels bearing ovules 

 only at base, and forming a consolidated common placenta 

 in the axis. Mitella and Dionsea help out this conception. 



312. One-celled, with. Parietal Placentae. In this not 

 uncommon case it is conceived that the two or three or 

 more carpel-leaves of such a compound pistil coalesce by 

 their adjacent edges, just as sepal-leaves do to form a gamo- 



sepalons calyx, 

 or petfds to form 

 a gamopetalous 

 corolla, and as 

 is shown in the 

 diagram. Fig. 

 333, and in an 

 actual cross-sec- 

 tion, Fig. 334. Here each carpel is an open leaf, or with some introflexion, 

 bearing ovules along its margins ; and each placenta consists of the con- 



FlG. 331, 332. Pistil of a Sandwort, with vertical and transverse section of the 

 ovary : free central placenta. 



Fig. 333. Plan of a one-celled ovary of three carpel-leaves, with parietal pla- 

 centse, cut across below, where it is complete ; the upper part showing the top of 

 the three leaves it is composed of, approaching, but not united. 



Fig. 334. Cross section of the ovary of Frost- weed (Helianthemum), with three 

 parietal placentae, bearing ovules. 



Fig. 335. Cross section of an ovary of Hypericum graveolens, the three large pla- 

 centse meeting in the centre, so as to form a three-celled ovary. 336. Same in fruitj 

 the placentte now separate and rounded. 



