ИСТИНА

Recent studies found morphogenetic plasticity of inflorescence tips in several ba- sal monocots, including Potamogetonaceae. Spikes of Potamogeton spontaneously produce terminal flower-like structures with unstable morphology that sometimes can be interpreted as pseudanthia. This phenomenon has been explored in two species with small flower-subtending bracts. We increased taxon sampling to in- vestigate variations of inflorescence tip morphology and vascular anatomy in dif- ferent genera of Potamogetonaceae (Potamogeton, Stuckenia, Groenlandia). In Potamogeton, we studied species with different flower number per inflorescence and with flower-subtending bracts either present or absent. In Potamogeton and Stuckenia, certain plants produce a terminal flower-like structure at the inflores- cence tips while terminal structures were not found in Groenlandia. Our study shows presence of conspicuous wide flower-subtending bracts in Stuckenia pecti- nata, though they consist of two cell layers and lack vascular supply like the small bracts in Potamogeton. In S. pectinata, when terminal flower-like structure is ab- sent, inflorescences always develop a tubular structure with serrate edges (a fusion product of uppermost bracts that do not subtend flowers). We found differences in vasculature of inflorescence axis between many-flowered and few-flowered species of Potamogetonaceae. In many-flowered Potamogeton praelongus, inflorescence axis contains 4-8 vascular bundles. Below a node, each bundle divides into three bundles: central bundle supplies a lateral flower and two bundles fuse with simi- lar lateral braches of adjacent bundles. Terminal flower or flower-like structure is usually supplied by several (rather than one) bundles derived the bundles of the inflorescence axis. In few-flowered species (Potamogeton crispus and Groenlandia densa), bundles of inflorescence axis divide several times and form a ring from which organs of the uppermost (or the only two) flowers are supplied individually. In the absence of a single vascular trace, the lateral flowers of the few-flowered spe- cies resemble the terminal flowers (or flower-like structures) of the many-flowered species. Stuckenia has a more reduced vascular system in the inflorescence axis. It consists of a concentric vascular strand with 1 – 2 xylem elements in the centre. Lateral flowers are supplied by direct radial branches of this strand. In inflorescence vasculature, Stuckenia resembles Ruppia (Ruppiaceae). Patterns of inflorescence vasculature do not depend on presence or absence and size of flower-subtending bracts. The work is supported by RFBR, project № 14-04-31271.