Sexual reproduction in angiosperms was once a taboo sbject. When plants were first classified it was considered very rude (by the stuffy Victorian sponsors of the great plantsmen) to call flower parts anything to do with sex so we have names like stigma, anther, style and others in order to avoid calling the sexual organs just that.
However, even if the Victorians may have confused or even delayed botanists’ investigation of the sexual reproduction in angiosperms we have thrown off the cautionary mantle and now there is nothing private about a plant’s sexual organs.
To explain , in angiosperms, the flowers can be single sex like holly (Ilex) (having only the male or female sexual parts) or hermaphrodite (having both). In either case male gametes are produced in organs called anthers and these are often borne on filaments or stalks. Inside the anthers are male gametes, carried in pollen grains. Pollen grains may be smooth (as in most wind pollinated plants) or they may have barbs or be sticky so they can attach to the hairs of insects or animals. Female gametes are made in organs called ova (in angiosperms, these are protected by the ovary). Above the ovary is the receptive organ which recieves the pollen and allows the male gamete to germinate, form a tube down it and then the male gamete enters the ovary where it fertilises.
Sexual reproduction occurs when the male gamete is tranferred from the anther to the ova and fertilises it.
Sexual reproduction in angiosperms includes two disctinct phases. First pollination where the male gamete in pollen is tranferred to the receptive part of the female sex organs. If the pollen is acceptable and the stigma is ready to be polinated a small tube grows from the pollen grain and travels down to the ovary. Then the male gamete travels along this to fertilise the egg. Pollination does not always result in fertilisation because if the male gamete is weak or the pollen not suitable, the stigma will not allow the pollen tube to germinate. Fertilization occurs when the male gamete reaches the ova and unites with it, forming a zygote, which is the first stage is seed production.
To make a gamete, cells divide by meiosis or the reduction division and each resulting cell has half the number of chromosomes of the parent plants. So, when the gamees unite they form the full chromosome number but in a combination which will be slightly different from the parent plant. This is the beauty of sexual reproduction because reproducing in asexual ways like budding, corms, rhizomes, cuttings etc means the new plants are clones of the parent plant but sexual preproduction allows the combination of the genetic material in such as way that beneficial characteristics may be found in new combinations.
Self pollnation occurs when the pollen is transferred from the anther of one flower to the stigma of the same flower or another flower of the same plant. Most flowers protect themselves against this because they wish to have genetic variation but many will allow it to happen if cross pollination has not occured over a given time so that they can at least produce some seed that season. They do this by first having the style over the anthers so any pollen dropped does not land on their own stigma and the stigma will not allow its own pollen to germinate. However, of self polination is allowed the anthers will grow above the stigma so that pollen falls on it and also the chemistry changes in the stigma to allow its own pollen to germinate.
Cross pollination is the transfer of pollen grains from the anther of one flower to the stigma of another flower on another plant of the same, or closely related species. Stigmas produce chemicals which stop the germination of pollen tubes from different species and so avoid cross genus crosses occuring.
Cross pollination involves a vector like wind, water or an insect and manypants produce scents and nectar in order to attract their pollinator. Some even flower at night to attract particular batsor moths and different shaped flowers have evolved in order to make pollination easier – along with the tempting offer of a drink of nectar as the animal’s reward for pollinating the flowers.
Plants are amazing things and will change their sexual behaviour to survive, as in the self pollination example. In angiosperms, the developing seeds are further protected by the swelling of the ovary to form a fruit in which the seeds are protected.
So plants are sexual beings, although perhaps not quite on the promiscuous levels of certain animal species I could mention!