THE COMPOUND PISTIL. 



293 



fectly, we have only to imagine two, three, or any number of pistil- 

 leaves (like Fig. 490), arranged in a circle, to unite with one another 

 by their contiguous edges, either without any intro- 

 flexion or infolding at all (Fig. 504), or at least 

 without their infolded edges having reached the cen- 

 tre and united there (Fig. 505, 506). The combina- 

 tion is accordingly much like that by which petals 

 unite to form a monopetalous corolla, only the edges 

 of the pistil-leaves are always turned in, where they 

 bear the ovules. Such an ovary may 

 well be compared with the valvate un- 

 opened calyx of Clematis, the' margins 

 of the sepals more or less turned in- 

 wards (Fig. 445). Every gi-adation is 

 found between axile and parietal pla- 

 507 centation, especially in the St. Johns- 



wort family (Fig. 508, 509) and in the Gourd family. 



555. An ovary with pai-ietal placentae is necessarily one-celled; 

 except it be divided by an anomalous partition, such as is found in 



Cruciferous plants, and in the Trum- 

 pet Creeper. 



556. It wiU be seen that parietal 



placentas are necessarily double, like 



the placenta of a simple ovary, or of 



each carpel of a compound several- 



' celled ovary ; but with this difference, 



that in these the two portions belong to the two margins of the 



same carpel ; while in parietal placentae they are formed from the 



coalescent margins of two adjacent carpels. 



557. The number of carpels of which a compound ovary consists 

 is indicated by the number of true dissepiments when these exist ; 

 or by the number of placentce, when these are parietal ; or by the 

 number of styles or stigmas, when these are not wholly united into 

 one body. Thus a simple pistil has a single cell, a single placenta. 



no. 506. Plan of a one-celled oyary with three parietal placentas, cut across below ; the 

 upper part showing the top of the three leaves it is composed of, approaching, but not united. 



FIG. 507. Ovary of Helianthemum Canadense, cut across, showing the ovules on three 

 parietal placentas. 



FIG. 508. Transverse section of the ovary of Hypericum graveolens ; the three large 

 placentae meeting in the centre, but not cohering. 509. Similar section of a ripe pod of the 

 same ; the placentae now evidently parietal. 



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