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Fruit Dispersal and Safe Storage

The Wild Almond Brabejum, a riverine species, has water-dispersed fruit which ripen in March and are washed down river during autumn and winter floods. Fruit germinate immediately on suitable substrates. Since riverine habitats are unstable and offer a short-lived germination substratum, the fruit are short-lived. Any seedlings which are not firmly rooted by the first winter rains will be washed to sea. Protea Atlas Logo

The other proteas have fruit of two types. In Cape Floral species these can broadly be classified as serotinous and myrmecochorus - both adaptations to the need to store nutrient-rich seeds safely in a fire-prone, nutrient-poor environment where seedlings stand little chance competing with adults for nutrients.

(1) Serotinous fruit are soft-shelled nutlets, flat or cylindrical in shape, often with hairs or wings. These are found in Aulax, half of Leucadendron species and Protea. In most Cape Floral members of these genera fruit are retained with the flower-heads on the plant (serotiny), after a lengthy ripening period of about 7 months. These seed-heads are fire-safe and only release their fruit when the water supply to the seed-head stops. This normally only occurs when plants are killed by fire or die, or when insects consume the stalk of the seed-head. Thus fruit accumulate on the plant from year to year, and the entire fruit crop (minus that destroyed by insect predation) is stored on the plant. In the savannas where fires tend to be annual, and in dry areas where fires may not occur for centuries, fruit of Protea, Faurea and certain Leucadendron species are dropped onto the ground after a two to four month ripening period and are not stored on the plants.

The function of the fruit hairs is three-fold:

  1. In the opening seed-head they expand forcing the fruit free of the heads
  2. Yhey are buoyancy devices in high winds, but on touching the ground they function as grapnels, anchoring the fruit; and
  3. They position the fruit on the soil surface to ensure optimum water uptake for germination.

Serotinous species release their fruit onto the soil surface after a fire. Consequently, fruit are available as food to rodents and birds until they germinate after the winter rains. Serotinous species in the Fynbos are thus vulnerable to extinction should fires occur in spring. Summer and autumn fires are more favourable, as they reduce the period that fruit lie on the ground.

One serotinous species (Leucadendron platyspermum) has got around this predation problem - its fruit are retained in the cones until after the autumn rains initiate germination: the growing root then pushes the fruit out of the cone.

(2) Myrmecochorous fruit are relatively hard-shelled nutlets, which are rounded or ovoid in shape and usually covered with a fleshy skin (the antfruit or elaiosome). Often called the Leucospermum fruit type, it also occurs in Serruria, Spatalla, Sorocephalus, Orothamnus, Diastalla, Vexatorella, Paranomus and half of Leucadendron species. This fleshy skin is why Pincushions are called Leuco (white) spermum (seed), it is the white antfruit which gives the fruit its colour, when this is removed the fruit is black underneath, and hard and smooth.

The fleshy skin is thin - too thin to attract birds as seed dispersers. After a 6-8 week ripening period, the antfruit secretes an ant alarm-pheromone, which attracts ants. The ants carry the seeds (sometimes removing them from the seed-heads) to their nests where they consume the antfruit. This leaves a hard, smooth nutlet, with no purchase for removal from ant's nests. The seeds, very much in demand as a nutrient-rich food source, are thus safely buried from predators. However, the invasive Argentinian Ant Iridomyrmex humilis may have upset the entire relationship by consuming the antfruit without burying them, thereby leaving seeds exposed to predation. Whether extinction of myrmecochorous proteas will occur where Argentinian ants are common remains to be seen.

Because myrmecochorous fruit are buried in the soil, they do not require any hairs to position them for water absorption (and hairs would allow ants to discard fruit from their nests). They do however use subtle clues to determine when the above ground plants have been killed by fire and thus that conditions are suitable for germination. Growing these species from seed requires that these cues are understood. Many people have given up trying to grow these species from seeds because they have not learnt to provide the correct cues. Have a look at Seed Sizes and Growing Proteas.

While by far the majority of proteas are sensible, some 24 Conebush species are utterly silly. They just throw their precious seeds onto the ground. They have no antfruit, no protection and – in the nutrient-poor Fynbos – no future. What to you think they are trying to do? Hint - have a look at Mystery Solved.

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