Diverse ways of pollination, including aerial, epihydrophilous and hypohy-
drophilous ones occur in the Najadales. Although the knowledge of pollination biology in this
order has been largely accumulated in recent years, most researchers ignored the role of stig-
mas in the pollination process. Based on the previous works, we supplemented observations
on stigmatic characters and pollination mechanism in nine species from four families. The
flowers of Aponogeton lakhonensis ( Aponogetonaceae ), Triglochin palustre ( Juncagi-
naceae), Potamogeton crispus, P. distinctus, P. gramineus, P. pectinatus, P. perfo-
liatus, P. pusillus (Potamogetonaceae), are all found to open above or on water surface.
Stigmas in these species have a large surface area. Correspondingly, the pollen grains are all
found to be globular. Of the eight species, except for P. pectinatus and P. pusillus, the
remaining six obviously expose their stigmas out of the perianth. Another species , Najas
marina (Najadaceae), has filiform stigmas and ellipsoidal pollen grains which can germinate
in water before being transferred onto stigmas in natural conditions. Pollination takes place in
this species strictly under water. The characters of pollen and stigmas could be divided into
two types, and the pollination system is of two kinds of mechanisms of stigmas capturing
pollen in the Najadales. The species with globular pollen grains have wide, large stigmas
while those producing filiform or precocious pollen grains, which are likely to be captured by
stigmas during the pollen dispersal by water currents, normally have filiform stigmas. It is
inferred that various water stresses might have resulted in the diversification of pollination
system in the Najadales.
