210 



MORPHOLOGY OF THE ANGIOSPERMS 



two primitive families, with placentation transitional from laminar to 

 submarginal, seem to show that the primitive ovule position was 

 laminar and that the submarginal type has been derived from the 

 laminar by restriction of ovules to the near-marginal areas. 



A similar evolutionary step is evident in the Nymphaeaceae, Cabom- 

 baceae, and Ceratophyllaceae, where ovule number is reduced in the 

 more specialized genera — Niipliar, Brosenia, Cobomba, CeratophijUum, 

 Nelumbo. In most of the genera of the Nymphaeaceae, the ovules are 

 numerous and distributed over the lamina; in Brasenia, there are only 



Fig. 84. Diagrams of cross sections of syncarpous ovaries showing varieties of 

 placentation by differences in degree of closure of constituent carpels. A, C to F, 

 forms of parietal placentation, by union of open or partly closed carpels; fusion 

 between adjacent carpels by margins only in C, E, F; by margins and sides in 

 A, D; B, axile placentation, by union of closed carpels. A, Tofieldia calyculata; B, 

 Lilium regale; C, Platystemon sp.; D, Aristolochia clematitis; E, Argemone mexi- 

 cana; F, Reseda lutea. {After Jtihnke and Winkler.) 



two ovules, described in taxonomic treatments as "dorsal," because they 

 are borne close to the midvein. But they derive their vascular supply 

 from anastomosing branchlets from both ventral and dorsal bundles 

 (Fig. 85A), as do most of the ovules in typical laminar placentation. 

 Cabomba has four, sometimes three, ovules (Fig. 85B). Where there 

 are four, all are attached between the dorsal and ventral bundles, as in 

 Brasenia. Where there are three, the third is on the ventral side and 

 above the others, and the vascular traces of all three come from the 

 ventral bundles: those to the lower ovules as laminar branches across 

 the carpel sides, that of the upper ovules, dii-ectly. Ovule position and 

 vascular supply in these two genera clearly indicate derivation from 

 laminar placentation, as would be expected in these highly specialized 

 genera of the nymphaeacean line. The carpel of CeratophijUum, with 



