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Afrikaanse Inligting
Plants with Flowerheads that Look Like Flowers
In proteas and daisies the "blossoms" are heads
containing masses of flowers. In vygies each head is a single flower. The floral
diagrams below outline the salient features of the three groups.
The Daisy or Sunflower family (Asteraceae or Compositae) is one of the largest families of flowering plants, containing nearly 25 000 species (i.e. 10% of plant species). Examples include food plants: artichokes, chicory, lettuce, sunflower; decorative plants: chrysanthemums, dahlias; and horrible weeds: cosmos, dandelions and thistles. Members of the family are characterized by the presence of resin canals or latex ducts in the leaves and stems; by the use of inulin instead of starch to store energy in the roots; and by the presence of fatty oils in the seeds. More importantly the florets are grouped into a capitulum (head) containing many florets surrounded by an involucre of protective bracts. Furthermore the floret has a single-chambered ovary containing a single basal ovule, which becomes a single seed dispersed in the ovary (technically an achene). Furthermore, the ovary is inferior (the sepal and petal bases are fused with the ovary wall and appear to arise above the ovary). The five petals are fused to form a tube. Most interesting is the mode of pollen presentation: the five anthers are fused into a cylinder and, when ripe, release the pollen into the hollow of the tube. The style then elongates through the tube pressing the pollen out so that it can be collected by pollinators. This `male stage' gives rise to the `female stage' when the stigma splits into two lobes exposing the stigmatic surfaces which collect pollen from passing pollinators. Should there not be any pollinators available the style arms curve back on themselves and collect pollen resulting in self-fertilization. In some species certain florets may be only male or female or self-incompatible and thus cannot achieve self-fertilization.
The Protea family has very variable inflorescence structures, and is most reliably identified by the florets. The ovary is superior (situated above the point of attachment of the other floral parts) and contains one chamber bearing one or two (occ. many) ovules. The four (never five) tepals are usually fused in various combinations for part of their length and have cupped tips which bear the four almost stalkless anthers. At the base between the tepals and the ovary are four scales (called hypogynous scales or nectaries). Now the term `tepals' is used when petals and sepals are not distinguishable. Typically in plants the outer ring of parts are the sepals, followed by petals and anthers progressively inwards. Thus the tepals in protea may be petals with the outer sepals lost. But typically the parts in each ring are offset to occur in the gaps of the adjacent rings. In proteas the anthers are not offset relative to the tepals: thus the latter must be the sepals and possibly the scales (which are offset relative to the tepals) are the remains of the petals. This issue is still the subject of contentious debate. A feature of the Protea family is the mode of pollen release. The style is a long, wiry structure which becomes tightly bent as the floret grows. Before opening, the pollen from the anthers is released onto a conspicuously thickened portion of the style, called the pollen presenter. The tension between the style and anthers is released when a pollinator probes the florets, resulting in the tepals releasing the style which springs out at the pollinator. Should no pollinators visit the flowers the tension results in the release of the styles later during the day. This is the male phase. Pollen not removed drops off in the heat of the day. Thereafter follows the female phase where the stigma - a tiny groove at the tip of the style - opens up and becomes receptive awaiting pollen from pollinators. A feature of the Protea family is that very few of the florets (1-15%) normally produce fruit.
Vygies have the entire blossom consisting of a single flower. The sepals are usually five in number. However the petals, which are not petals at all, but modified stamens, are numerous and resemble the disk florets of daisies. The anthers are also numerous. In the centre is a five-lobed style, with a semi-inferior ovary containing five chambers with many ovules in each. Interestingly, the seed-head is one of the most complicated fruit structures in the flowering plants. Why? Because they are serotinous, storing the seeds. Seeds are not stored for fire protection as in the Proteaceae, but for release after suitable rains - a handy adaptation under desert conditions. The complicated valves of the dry seed capsules open up during heavy rains and seeds are washed out by raindrops. After the rain the valves of the drying capsule close, storing the remaining seeds for future rains. Of the over 2000 species in the Vygie family 99 per cent are confined to southern Africa's Karoo, Desert and Fynbos regions.
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