Flowers are the sexual organs of the plant and the initial stage in its reproduction. The flower’s purpose is to enable the pollination process to take place. Pollination involves the transfer of pollen from the anther to the stigma and fertilisation of the ovule. Following pollination the fertilised embryo develops into the seed and the surrounding tissue into the fruit.
The stamen and the stigma may be in close proximity on the same flower, or in a few species, on separate flowers, on separate plants. To improve genetic variability in progeny it is preferable for pollen to reach stigmas on separate genetically different plants. To get the pollen from the stamen to the stigma, plants have evolved a number of ways, generally involving external factors such as wind or interactions with animals. Some plant species may be self-pollinating.
Successful seed-set needs pollination, but pollinated flowers may not necessarily lead to successful seed set. Adverse weather conditions, inadequate rainfall, very hot days etc may lead to failure to set seed.
In some species wind is used as the pollinating vector e.g. Plantago gaudichaudii produces clouds of pollen when knocked. Wind-pollination appears to be fairly wasteful of large amounts of pollen but presumably works well for species with large numbers of relatively nearby individuals, which mature more or less simultaneously. Grasses as a group are wind-pollinated – this explains why they have such unobtrusive flowers – they don’t need to draw anyone’s attention. The anthers may be the most colourful part of the flower.
Animal-plant interactions would appear to be more efficient in getting pollen to more sparsely dispersed individuals. For this however plants pay a price, having to provide an attractant such as nectar as food for the animal. Little is known about the pollination of most of our native woodland species.
Orchids are renowned for their intricate pollinator partnerships. The woodland orchid Pterostylis curta is pollinated by pseudocopulation of male fungus gnats (Mycetophilidae), while Dipodium punctatum is pollinated by the native bee Chalicodoma derelicta. These orchids are very rare in our woodland and we have no observations on pollination but have noted some seed set.
Another family with particularly specialized flowers is Asclepiadaceae whose pollination mechanism may be conspicuously specialized (involving trapping of insects' (often flies) legs or probosces between the osmotically elastic anther wings, and withdrawal entailing capture of the pollinia by means of `sutured corpuscular pollen carriers'). Species in the woodland include Marsdenia viridiflora and Marsdenia rostrata, the exotic *Gomphocarpus fruticosus and the closely related Parsonsia straminea (Apocynaceae).
Other species are probably pollinated by native bees e.g. Dianella longifolia, Solanum cinereum while the introduced Honey bee *Apis mellifera is very common and visits both exotic and native species, though whether it helps or hinders seed-set in native species is not known.
Flying foxes may visit the woodland at night, perhaps pollinating the Eucalyptus species. Birds may be involved in some pollination though we have not identified any particular species for plants in our woodland. Birds are reported to pollinate many native species in other vegetation types such as Grevillea in sandstone woodlands but these species are not found in our woodland, though Grevillea juniperina occurs in Cumberland Plain Woodland near Blacktown.
Asterisk * indicates exotic species naturalised at the Australian Botanic Garden.