Morphology of the Gynobase in Mertensia

Abstract

The conformation of the ring-like disc or gynobase surrounding the gynoecium varies greatly among the genera of Boraginaceae. The morphology of the gynobase ha's been taken for granted by most systematists, and has been used descriptively in connection with description of nutlet attachment. No morphological or evolutionary significance has been attached to the gynobase. In other families, anatomical and morphological studies have demonstrated that many discs, nuptial nectaries, and floral squamellae may be interpreted in terms of the other appendages of the flower. For example, it is believed that the sheathing disc about the carpel in many Leguminosae represents an abortive set of stamens.1 Structures adjacent to the gynoecium which have been observed in many Apocynaceae are interpreted as carpellodes.2 The heteromorphic nature of the gynobase of Mertensia is apparent on gross examination (Figs. 15, 16). The four unequal lobes of the gynobase alternate with the four nutlets of the pistil. Opposed to the carpels are two small obtusely triangular pointed lobes. Alternated with the carpels are the two large lobes which equal or surpass the nutlets in size. For a better understanding of the morphology of the gynobase it is necessary to describe briefly the anatomy of the flower in Mertensia. The vascular anatomy of the flower is not especially complicated in Mertensia. In most respects the description given here applies to other members of the Boraginaceae which I have examined (Figs. 1-13). The pedicel contains a circle of ten strands, alternately large and small (Fig. 1). The five smaller traces swing outward and upward to supply the sepals (Fig. 4). These strands split radially into 3 parts after they leave the stele. Lateral strands of adjacent calyx lobes do not fuse. The sepal gaps close, and then the petal and stamen traces begin to depart (Fig. 5). Each stamen receives one trace. There are five petal traces, and these split radially into three members after leaving the stele. The petal and stamien trace gaps close and the stele becomes provascular except for the strands which are to supply the carpels (Fig. 7). Each carpel is provided with three traces. The two ventrals of each carpel swing inward and upward toward the center of the flower, each eventually ascending through the placenta into the funiculus (Figs. 10-12). The dorsal bundle pursues a devious upward course which leads into the style. The median lateral bundles seen in the nutlet walls apparently arise from the placental bundles, curve upward into the wall of the nutlet and then gradually die out.