FLOWERS AND SEX
Self-pollination is the transfer of pollen from the anthers to the stigma of the same flower or to a flower on the same plant. This results in inbreeding and reduction of variability but may still result in viable seeds. Self pollination is most likely in bisexual flowers where male and female parts are close together. About 10% of species reduce the chance of self pollination by completely separating male and female parts in the form of unisexual flowers. These can be on the same plant (Monoecious) or different plants (Dioecious). Often unisexual flowers with no petals are clustered on catkins but some plants have unisexual flowers with petals as in the Red and White Campions and the Holly tree.
Some plants have male flowers on one plant and female flowers on a different plant. These are known as dioecious. (Greek for 2 households). The first photo is of two Male Black Poplar catkins. The male flowers are arranged spirally all the way down the catkins. Each male flower consists of several red anthers which will eventually split to release pollen grains. Male catkins generally release pollen before the leaves come out to aid pollen dispersion on the wind. Female flowers are on catkins on a different tree and are shown in the second photo, which is of a Grey Poplar. Each female flower has a green ovary and a red forked stigma. Wind delivers pollen to the stigmas and pollinates the flowers. In May the ovaries will become fruits full of seeds with long white hairs, which aid wind dispersal. Other dioecious species include Box Elder, Ginkgo, Holly, Juniper, Bay, Yew, Stinging Nettle and Red Campion. Ash trees may be monoecious or dioecious.
Some plants have male and female flower parts that are separated but on the same plant. These plants are known as monoecious. The Common Hazel is a monoecious tree. The male flowers are on yellow catkins that hang down ready to release pollen onto the wind (first photo). There may be over 200 unisexual male flowers on a single catkin. The catkin is covered in scales. Two male flowers are located beneath each scale. Each flower has four anthers. The scale lifts and the anthers split open to release pollen when conditions are favourable for pollen dispersion. After pollen release, the male catkin soon drops off. The female flowers are red and tiny and are located on the branch above the catkin. They are shown in the close-up in the second photo. The red stigmas collect pollen from the wind and eventually the flower forms a hazel nut. Alder, Oak, Beech, Birch, Hornbeam and Sweet Chestnut are other examples of monoecious trees.