Many plants have evolved complex sexual reproductive systems, which is expressed in different combinations of their reproductive organs.
Alnus serrulata has unisexual flowers and is monoecious. Shown here: maturing male flower catkins on the right, last year's female catkins on the left - wikipedia 
Some species have separate male and female plants, and some have separate male and female flowers on the same plant, but the majority of plants have both male and female parts in the same flower - wikipedia 
Some plants change their morphological expression depending on a number of factors like age, time of day, or because of environmental conditions. Plant Sexual Morphology also varies within different populations of some species.