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Afrikaanse Inligting
Pollination of Proteas
The majority of proteas
flower in spring and summer, with fewest species flowering in autumn. This pattern may
relate to the abundance of pollinators, optimal conditions for nectar secretion, or the
need for myrmecochorous species to synchronize seed release to the season of maximum ant
activity.
Have a look at Timing of Flowering and Growth.
A feature of protea pollination is that pollen is deposited by the anthers onto a modified tip of the style (called the pollen presenter). Flowers usually remain closed until triggered by a visiting bird, mammal or insect, when they snap open releasing the style which rubs pollen onto the visitor. If no visitors arrive, flowers may open during the heat of day (or late afternoon in mouse-pollinated species). Pollen falls off the pollen presenter after a few hours of exposure to air.
The stigmatic grooves of proteas are closed when flowers open, but open maximally after 24 to 36 hours. The stigma is then ready to receive pollen brought by visitors. Thus proteas are protandrous: the male organs mature before the female and prevent self-pollination. Most Protea species seem incapable of self-pollination (i.e. are self-incompatible), although certain Serruria and Leucospermum species do set seed when pollinated with pollen from the same plant. Dioecious proteas (i.e. Leucadendron and Aulax with separate sexes) are obviously unable to pollinate themselves.
Flower-heads may remain open for a few days up to several weeks. Flowers almost always open from the outside towards the middle of a flowerhead. However, when flowerheads are grouped into conflorescences, inner heads may open before the outer.
There are four main syndromes of pollination in our proteas:
- Rodent pollination: Several species of gerbils, mice, rats and shrews visit flower-heads of some Protea species and Hook Pincushions. Rodents are attracted by a strong musty odour, and a reward of syrupy sugar which is secreted in large quantities. In order to prevent birds and insects from stealing this nectar, rodent-pollinated (therophilous) proteas have inconspicuous brown or black involucral bracts. Flower-heads are usually hidden inside the bush at ground level, where they are accessible to rodents. The insides of the involucral bracts may be pale white and the tips of the flowers may be shiny red - both serve to guide the rodent to the nectar in the dark. The nectar is contained within the tepal tube, and the distance between the pollen presenter and the nectar is the same as the length of the rodents snout (about 10 mm) allowing pollen to be deposited on the head. The nectar is rich in cane sugar (sucrose). Since most therophilous proteas flower in spring, nectar is available during the rodents' breeding period.
- Bird pollination: In contrast to therophily, flower-heads of bird-pollinated (ornithophilous) proteas are often brightly coloured. Many Mimetes, Protea and Leucospermum species are ornithophilous. Because bird and human vision is similar, ornithophilous flowerheads are aesthetically pleasing to us. The bright red, orange, yellow, green and pink colouring on involucral bracts and styles serve as visual attraction. The colours are relatively inconspicuous to many insects which cannot see red and orange. Since birds do not rely on smell, ornithophilous proteas have little odour. A perch may be provided by stems, unopened florets, and awns, whereas, in some species, birds have to sit on the ground. Copious quantities of easily digested glucose-rich (fruit sugar) nectar are secreted. The most important bird visitors are the Sugarbirds, which can truly be considered Protea-birds, and the Sunbirds.
- Insect pollination: Many of the "bird-pollinated" proteas are visited by large numbers of beetles - up to 2000 insects may occur in a single flower-head. Among the most important of these are the Scarab Beetles (Scarabaeoidea: e.g. Protea Beetle Trichostetha fascicularis and Monkey Beetles) and Rover (Staphylinid) beetles. The relative importance of birds and insects in pollinating these species is unknown. Although insects in Protea flower-heads often end up as bird-food, they adequately pollinate flowers when birds are absent. Another feature of "bird-pollinated" proteas is the occurrence of thousands of itch mites (Proctolaelaps vandenbergi) in flower-heads. These mites are phoresic (transported) on birds and beetles and feed on nectar and pollen. Although absent on young flower-heads, mature Protea flower-heads may contain over 6000 breeding mites. Whether these mites might play a role in pollination is not known. Because bird-pollinated flower-heads are relatively odourless itch mites may provide endless entertainment: merely watch the reactions of someone smelling a bird-pollinated Protea flower-head too closely! However, certain proteas are exclusively visited by insects: Smaller Protea (the Shale Proteas) and Leucospermum species are insect pollinated, mainly by bees and wasps; Leucadendron species are visted by a number of beetles; and most of the smaller genera are visited by a variety of beetles, flies and wasps. These species can usually be distinguished by their production of sweet, spicy or sour odours, and relatively meagre production of nectar. They tend to be pink, cream or yellow in colour, and are usually considerably smaller in size than bird-pollinated species.
- Wind pollination: There are only 10 wind-pollinated proteas in southern Africa: all are Leucadendron species. These are characterized by not secreting nectar - most do not even have nectaries - and being odourless. The females have large stigmas for filtering pollen out of the air. In the males the pollen does not adhere to the pollen presenter, but is scattered in large showers on opening, from whence it drifts in the wind.
Have a look at Pollinators on Proteas
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