Pistillate and Staminate
A flower is pistillate if it possesses only pistils, which are a plant's female reproductive structures. If it possesses only stamens (the male reproductive structures), it is called staminate.
Monoecious and Dioecious
The
Alder is
monoecious. Shown here: maturing male flower catkins on right, last year's female catkins on left
Holly (
Ilex aquifolium) is dioecious: (above) shoot with flowers from male plant; (top right) male flower enlarged, showing stamens with pollen and reduced, sterile stigma; (below) shoot with flowers from female plant; (lower right) female flower enlarged, showing stigma
and reduced, sterile stamens with no pollen