During the boreal autumn, while in South Africa it is spring, in the greenhouses in which the most sensitive specimens of the collections of the Royal Botanical Garden of the University of Alcalá, carrion odors appear as unpleasant to our nose as they are attractive to carrion flies.
Living in the arid areas of the Mediterranean climate of South Africa, in the biome of the fynbos, and in coastal deserts of Namibia it is not easy at all. Winters are colder than in any other low-lying African zone, frosts are frequent during the rainy season between June and August. In addition, the dry season exceeds eight months with rainfall that sometimes does not occur for years.
The strategies that plants have developed to survive in these conditions are many and not in vain some of the most extraordinary plants in the world, including more than 6,000 endemic species, live around the Cape region and on the desert west coast of Namibia.
How Flowers Survive in Arid Deserts
To survive and grow in an environment where water is scarce, South African milkweed has developed the same vital strategy as other arid zone plants: succulence. In succulent plants some organ or tissue is modified to allow the storage of water in large quantities. This allows them to survive in arid and dry environments uninhabitable for other plants. In addition, water reserves are saved thanks to a special photosynthetic pathway.
The same happens with some apocinaceae of the genres Huernia, Orbea, Piaranthus or Stapelia that I have included in the composition of the photographs in Figure 1.
Every year, during the southern spring, South Africa is the scene of an astonishing phenomenon: the sudden and spectacular flowering of the fynbos. A vast expanse of wild plants with multi-colored hues. A few drops of rain are enough to transform the arid landscape into a huge carpet of shrubs and herbaceous plants full of flowers. Hundreds of different species, millions of individuals, open their flowers in a vital explosion that seeks to attract pollinating insects.
The supply is extraordinary and the insects, although they are not lacking, are unable to meet the demand. This is the time for specialist plants capable of making a differentiated offer.
The apocinaceae that inhabit arid South African zones compete with other plants in terms of the color and beauty of their flowers. But that is not enough when after the rains an immense tapestry of plants unfolds, each more striking. Milkweed flies disdain the huge array of insects of all kinds and concentrate on a few, few but effective: carrion flies.
Its flowers deceive flies by sight and smell. The attractive colored surface and fleshy texture of the petals mimic a decomposing dead animal. The flower emits an intense rotten meat stench that attracts flies that feed on animal carcasses.
Because they produce flowers at ground level that look and smell like decaying organic tissues, these milkweed, as well as do some very curious orchids that live with them, are an extraordinary claim for necrophiliac flies, which, moved by the irresistible biological imperative of reproduction, do not discriminate between a rotten corpse and a flower that smells of cadaverine.
That is the trick. The flies land on the flower thinking that they have found a place to lay their eggs. They move within the flower and collect or deposit pollen in the process. Unfortunately for them, their larvae are doomed. Although the mothers find nectar in abundance, there is no food for the larvae to feed once the flowers wither.
A bag of pollen
Milkweed flowers are very complex and characteristic. Unlike pollen in most flowers, which is released from the anthers while they are still attached to the flower, in milkweed the pollen remains inside a bag (polynya) until it comes into contact with the stigma of another flower of the same species. Among the more than 250,000 flowering plants, only orchids, other extraordinarily complex flowering plants, pack their pollen in a similar way.
The only way for pollinia to escape their chambers is for insects to extract them. Asclepiadaceae are entomophilous, which means that their pollen is carried by different species of insects. Most insects look for nectar, which they find in abundance thanks to the nectariferous glands at the bottom of the flowers. In the case of fly-pollinated milkweed (dipterophiles), in addition to nectar, the visiting Diptera are also looking for an appetizing place to lay their eggs and raise their larvae.
Whatever they are looking for, while moving over a flower the insects go from nectary to nectary until they remove the entire polinarium (the viscous corpuscle, two transfer arms and two polynyas) from the floral chamber in which they were enclosed (Figure 3). Once released, the insect does not teach and continues its search from flower to flower, so it very frequently accumulates multiple polinars, sometimes hooked in chains of ten or more of them, hanging from the animal’s leg.
What about the polynyas? Many will fall off as the insect moves. Some will find their destiny in another flower. In this case, pollination is a wonderful process. For simplicity, the polynya acts like a key that is inserted into a slot that, like a lock, prevents entry to the stigmatic chamber where the ovules await. As the insect flails, the arm of the translator breaks and the polynya remains inside the chamber.
When the polynya enters the chamber it begins to swell. In a few hours, it opens through a germination crest from which multiple pollen tubes emerge, each from a pollen grain. The tubes grow and penetrate one of the two ovaries of each flower, each of which can contain up to 200 ovules, which will be fertilized by the male gamete transported within the pollen tube.
The fertilized eggs will transform into seeds that ensure offspring in one of the most hostile environments on Earth.