Ontogeny, morphology, and systematic significance of glochidiate and winged fruits of Cynoglosseae and Eritrichieae (Boraginaceae)
Hilger, Hartmut H.
Plant Diversity and Evolution Volume 131 No. 3 (2014), p. 167 - 214
published: Apr 1, 2014
The development of flowers and fruits of 23 species of the Cynoglosseae and Eritrichieae, formerly for the most part included in the genus “Echinospermum”, was studied morphologically and histologically by means of SEM and microtome sectioning. 1. The early floral ontogeny is strongly uniform in all species (as well as in two species of the Lithospermeae investigated in addition). The sepal, petal, and stamen primordia display a regular 2/5 spiral phyllotaxis. The two carpels develop very late, when the stamina already exhibit the thecae. Soon the flanks of the carpel primordia fuse and give rise to a gynoecial cone. 2. The four mericarps arise as lateral bulges of the carpels. Thereby the gynoecial cone splits up into a terminal style, an intermediate mericarp-bearing columella, and a basal disk. The latter will apparently act as nectary. The part of the fruit that remains on the plant when the nutlets are released is termed gynobase. 3. Just before the stylar slit closes, a low transverse ridge divides the basal cavity of the gynoecium into two shallow grooves. This ridge is interpreted as the septum. The four ovules develop within the angle between the septum and carpel wall, but are clearly located on the septum. Therefore the placentation is (marginal-)axile. In later developmental stages the funiculi remain united with the carpel walls by intercalary growth, feigning a parietal placentation. 4. From the very first the ovules grow diagonally upwards into the mericarpial cavities. Inside the gynoecium four apical and basal septa narrow the connections between ovarial and mericarpial chambers, leaving only a small tube for each funicle. The abscission layers develop inside these massive diaphragms. 5. False septa, often considered characteristic in the Boraginaceae, do not develop in all species. If present, they originate from subepidermal ventral meristems of the carpels. They are not necessary for the formation of the mericarps. 6. The initial globose shape of the Cynoglosseae nutlets remains until ripening. After bulging up from the gynoecial cone the gynobase grows in horizontal direction. So the nutlets attain a position with their disks parallel with the basal area of the gynoecium. The style seems to insert on the receptacle. A subsequent growth in vertical direction brings the mericarps to their final oblique position. In contrast, in the Eritrichieae a forced vertical growth of the gynobase produces the typical triangular shape and the continuously oblique position of the mericarps. Asperugo and Sclerocaryopsis confirm their isolated position within the Eritrichieae by unusual nutlet arrangement. Neighboring mericarps of different carpels turn to each other. 7. Most members of the Cynoglosseae and Eritrichieae exhibit a thickened rim or broad wing around the disk of the nutlets and/or glochids on the nutlet surface. It may be of systematic importance whether the wings are developed prior to glochids or vice versa. In the latter condition a subsequent peripheral growth of the nutlet margins may produce a broad glochidless rim analogous to a wing. After ontogenetic investigation, genera with similarly looking fruits can be distinguished. Heterocarpy as well as heteromericarpy displaying pairs with different wing and glochid equipment seem to have arisen independently in various genera. 8. A broadened definition for the terms “eremocarps, mericarps, and fruits (Klausen, Klausenfrüchte)” is proposed. It may also be applied to the fruits of some other taxa in which parts of the fruit enclosing seeds break away and leave a gynobase.