Morphology and development of spikelets and flowers in Cyperus and Pycreus (Cyperaceae)

Background : Pycreus, Kyllinga, and Queenslandiella cluster together with Cyperus within the Cyperus s. lat. clade, one of the two large clades in Cypereae. However, in contrast with Cyperus, they have laterally flattened pistils/nutlets. Pycreus, Kyllinga and Queenslandiella form morphologically well circumscribed independent genera. In the context of a broader systematic project to work out a well supported, evolution based taxonomy for Cyperus s. lat., we present in this paper general morphological and developmental data of species of Pycreus in comparison with three species of Cyperus, including C. laevigatus with dorsiventrally flattened nutlets. Approach : Freshly collected material was investigated using scanning electron microscopy (SEM) and light microscopy (LM). Special attention was given to spikelet and gynoecial development. Results : SE micrographs of all species studied show an indeterminate rachilla with distichously arranged glumes, each subtending a bisexual flower. In spikelets of C. capitatus and P pumilus, the proximal glume sometimes subtends a lateral spikelet instead of a flower. In the species of Pycreus studied, each flower sits in a cavity formed by the growth of the rachilla, which is congenitally fused with the wings of the glume of the higher, alternate flower. Glumes appear successively, each soon forming a flower primordium in its axil, which develops according to a general cyperoid ontogenetic pattern. In Pycreus, the stigma branches grow out from dorsiventrally positioned primordia. During gynoecium development, a hypogynous stalklet (gynophore) appears in all species studied. Conclusion : In spikelets of Pycreus, the rachilla and wings of the glumes are congenitally fused and consequently develop with epicaulescent displacements of the glumes resulting in typical spikelets with flowers in cavities. In spikelets of Cyperus, a similar though less pronounced development results in spikelets with zigzagging rachilla. The particular positions of the stigma branches in C. laevigatus and Pycreus are explained by the development of the gynoecium from an annular primordium, which facilitates shifts in localisation of the stigma primordia. Though we consider the combination of the typical spikelet ontogeny and the independently originated laterally flattened nutlets to be strong arguments in favour of a genus Pycreus, a phylogenetic confirmation that the taxon is monophyletic is an absolute, until now unfulfilled, condition. Moreover, the consequences for the giant genus Cyperus must be taken in consideration.