214 MORPHOLOGY OF THE ANGIOSPERMS 



reduction in ovule number to one. The primordium of the more primi- 

 tive carpels is crescent-shaped in cross section, and closure is onto- 

 genetic; that of advanced carpels is ring-shaped in cross section, and 

 the maturing carpel is tubular and its margins are united congenitally. 

 The ontogeny of many carpels combines these two methods. The 

 primordium, at first crescent-shaped, becomes ring-shaped later. The 

 carpel, so formed, is distally open, and closed only in late stages; it is 

 proximally tubular, arising closed. (A tubular corolla with petallike lobes 

 has similar ontogeny.) 



The so-called U-type placenta is far from primitive and cannot be 

 considered in any way a basic type. It is a high type of placenta, the 

 result of extreme reduction in carpel length and ovule number. Ac- 

 cording to the theory that the U-type is the basic kind of placenta, the 

 typical double-row, submarginal placenta is considered derived from 

 the U-shaped type. This is an example of reading a series from the 

 apparently simple to the complex, because the simple was accepted as 

 always primitive. The series obviously runs in the other direction; 

 achenes are surely highly specialized carpels. 



In carpels in which the primordium changes during ontogeny from 

 crescent-shaped to ring-shaped, the level of transition is a point or line 

 where the margins meet. This transverse connecting bit of margin has 

 been termed, under the peltate theory, the cross zone. It varies in 

 width with the form of the crescent-shaped primordium but is usually 

 very narrow. The peltate theory stresses the cross zone as of much 

 morphological importance, but its existence is incidental to the change 

 in primordium shape, the beginning of congenital fusion of the carpel 

 margins. Recognition of the zone as significant of basal form in carpel 

 morphology has led to far-reaching misinterpretations in structure. 



The cross zone has been considered a placenta because, as seen in 

 achenes, the solitary ovule is attached at that level, apparently on the 

 connecting bit of transverse margin. But the ovule is the lowest member 

 in the submarginal placenta of the ancestral follicle, as shown by 

 anatomy and by the presence, in Clematis and Anemone, of vestigial 

 ovules above the normal ovule. Placentation called U-form has also 

 been called lateral, median, median laminar, median lateral, ventral 

 median, and submarginal lateral. Obviously, it is a reduction form of 

 submarginal. In both apocarpous and syncarpous gynoecia, reduction 

 in ovule number introduces much difficulty in the interpretation of 

 ovule position and the terminology of placentation; comparative, ana- 

 tomical, and ontogenetic studies are essential in interpretations. 



Reduction in Laminar Placentation. The evolutionary history of placen- 

 tation is clearly one of reduction of ovules from many and indefinite 

 in number to much smaller numbers and, ultimately, to two or one. 



