236 MORPHOLOGY OF THE ANGIOSPERMS 



locular, syncarpous ovary with parietal placentation has been derived 

 from the multilocular with axile placentation, the lateral walls, with 

 their placentae, are considered retracted; in this way parietal placenta- 

 tion has been claimed to have been formed from axile. In axile placenta- 

 tion, the ventral bundles are inverted — in contrast with the dorsal 

 bundles (Fig. 88). If the margins and placentae are simply withdrawn 

 by narrowing of carpel sides, the bundles would remain inverted. But 

 in many — perhaps all — taxa with this type of ovary, the ventral bundles 

 are normally oriented like the dorsals — Dilleniaceae, Violaceae, Carica- 

 ceae, Passifloraceae, Hypericaceae. The orientation of the bundles of the 

 carpels is the same as that of open carpels. 



The evidence from ventral-bundle orientation supports the fusion-of- 

 open-carpels theory of origin of this parietal type of syncarpous 

 gynoecium. An opening up of closed carpels to form this type of ovary 

 would be a return to an ancestral condition, a step not taken in evolu- 

 tionary modification as interpreted under the "law of irreversibility." 



The existence of syncarpy among open carpels is evidence that con- 

 nation in the gynoecium began, in the history of angiosperms, before 

 closure of the carpel, or that the two kinds of fusion arose together. 

 Apparently, evolutionary advance in connation and carpel closure is 

 continuing in living plants. 



Connation of open carpels suggests that the taxa with parietal plac- 

 entation are less advanced than those with axile placentae where the 

 carpels are closed. In most classifications of angiosperms, the Parietales 

 are placed among the lower orders on the basis of other floral char- 

 acters. The recognition of parietal placentation as a primitive, not a 

 highly advanced, type, supports the generally accepted position of this 

 order. (Close relationships within the Parietales are not necessarily 

 suggested. ) 



Where, in a syncarpous unilocular ovary, the ovules are borne on a 

 mound or column in the center at the base of the common locule, free 

 from the lateral ovary walls, the placentation is well termed, descrip- 

 tively, free central (Fig. 88B). The nature of this type of placentation 

 has been much discussed, with strongly divergent opinions that involve 

 the nature of the carpel itself and the question of basic ovule position — 

 cauline or appendicular. It has been suggested that free central plac- 

 entation should be called free marginal. If the term were modified to 

 free siibmarginal, it would be morphologically correct, because it sug- 

 gests the correct morphological condition — a placenta consisting of 

 carpel borders freed from the lateral walls and united in the center of 

 the gynoecium ( sometimes clothing the projecting tip of the receptacle ) . 

 But the long-used term free central is simpler, and also suggests the 

 freedom of the borders from the lateral walls and tlie median position. 



