238 MORPHOLOGY OF THE ANGIOSPERMS 



present, it is sheathed by the carpel borders on which the ovules are 

 borne. 



Abundant proof of the nature of free central placentation is found in 

 both ontogeny and vascular anatomy. In some genera of the Caryophyl- 

 laceae and Portulacaceae, axile placentation is transformed during de- 

 velopment of the flower into free central; the lateral walls of the carpels 

 break down, leaving the united placental areas free, except at the 

 base (Fig. 89). Flower buds of many genera show all stages of this de- 

 velopment. In other genera, the breakdown is incomplete in the basal 

 part of the ovary, and transverse sections made there show a chambered 

 ovary with axile placentation, whereas those made in the upper part 

 show a single chamber with a free central placenta. Where the walls 

 have degenerated, radial sterile areas separate the clusters of ovules 

 and show each cluster to consist of two rows of ovules. The well-known 

 genera — Lychnis, Silene, and Dkinthus — show this well. In the section 

 Polycarpeae of the Caryophyllaceae, the carpel walls do not break 

 down. Part of this family has free central placentation; part has axile. 

 And even in a single genus, Lychnis, both types occur: L. alpina has 

 axile placentation; all others, free central, formed, ontogenetically, from 

 axile. In the flower bud of Lychnis and Dianthus, placentation is parietal 

 at the top of the ovary and axile elsewhere. Later, the parietal part 

 becomes axile, and the axile section, except the base, becomes free 

 central. No morphological line exists between these types. Vascular 

 anatomy supports this interpretation of the nature of free central plac- 

 entation. 



Of the two prominent families characterized by free central plac- 

 entation, the caryophylls and the primroses, the latter is more ad- 

 vanced in placental type, because, in most genera, there is little or no 

 evidence of ontogenetic loss of carpel walls; a few genera — Dodecath- 

 eon, Lysimachia, Steironcma, Samolus, and probably others — show on- 

 togenetic loss of carpel walls; in most genera, free central placentation 

 is congenitally established. 



Placentation with the ovules borne at the base of the locule in 

 syncarpous ovaries is basal. Basal is a reduction type, derived from 

 free central placentation, with the placenta reduced in size and the 

 ovules in number, to few or one. No line can be drawn between free 

 central and basal placentation; if the ovule or ovules are borne directly 

 on the floor of the ovary without a definite projecting placenta, the 

 placentation is considered basal. It may be derived from axile, as in 

 the Fagaceae; it may also be derived from parietal, as shown by the 

 Droseraceae. Drosera has parietal placentation; Dionaea, basal; and in 

 Drosophyllum, where the ovules are borne at the base of the ovary 

 walls, the placentation is intermediate between parietal and basal. Other 



