Placentation refers to the mode of ovule attachment on the ovary wall. The ovary in most flowering plants is a compound structure formed by the union of two or more carpels. The number of ovules found within the ovary, the number of carpels fusing to form the compound ovary, and the position of ovules within the ovary following the union of carpels have generated several types of placentation in flowering plants. Little has been written about the evolution of placentation since Puri’s (1952) classic paper nearly 65 years ago. The last comprehensive account was by Stebbins (1974), almost 40 years ago. Recently, evolution of placentation has been studied in a few angiosperm orders such as Malpighiales (Endress et al., 2013) and Vitales (Ickert-Bond et al., 2014), but an overall perspective across angiosperms, considering evolutionary relationships of taxa, is lacking.

Flowers in most of the early branching/early diverging lineages of angiosperms have one to many free carpels, each carpel with one or few ovules along the margins sealed by secretion (Figure 1Ba; Puri, 1952; Stebbins, 1974; Endress and Igersheim, 2000). Early in the evolution of angiosperms, the carpels fused to form a compound ovary (Figures 1Bb,h). The fusion of the carpels led to a bi- or plurilocular ovary, with the number of locules being defined by the number of carpels fusing with each other. In this bi- o plurilocular ovary, the ovules develop along the axis of the central column (Figure 1Bb), on fused margins of their own carpels. Both Puri (1952) and Stebbins (1974) traced the evolution of various types of placentation from this axile placentation (Figures 1Ab,Bb) and considered marginalplacentation as the simplest and most primitive type (Figure 1). Puri (1952) suggested that evolution of placentation in flowering plants is complex, progressing in several directions along at least five divergent pathways (Figure 1A), whereas Stebbins (1974) argued that placentation evolution in angiosperms is uniform, proceeding in a single direction along three parallel pathways (Figure 1B). In the first pathway, there has been a reduction in the number of ovules to one in each chamber and subsequently to a unilocular ovary with a few or one ovule with basal placentation (Figures 1Ad,Bd). The unilocular ovary with basal placentation evolved further into an ovary with free central placentation, a trend accompanied by an increase in the number of ovules (Figure 1Be). In the second pathway, the initial stages were the same as in the first. There was a decrease in the number of ovules to one in each chamber, but the ovules were pendulous rather than basal (Figure 1Bf). This was followed by the evolution of a unilocular ovary with one pendulous ovule (Figure 1Bg). The third pathway also led to a unilocular ovary, with ovules borne along the ridges or all along the ovary wall. This is known as parietal placentation (Figures 1Ae,Be). The number of ovules, and presumably seeds as well, are generally large.