Charles Darwin’s abominable mystery
Students are often confronted with the statement that the symbiosis between bees and flowers is a prime example of co-evolution, an evolution, which saw both groups appear and evolve together. This, however, has not happened. The origins of the flowering plants of the angiosperm lineage that subsequently began to dominate the terrestrial flora, is known to have roughly coincided with the beginning of the Cretaceous Period, i.e. approx. 130 million years ago. Social bees, however, did not appear until about 30 million years ago in the Oligocene era of the Tertiary. The discrepancy prompted Darwin to make his famous utterance of the “abominable mystery of the angiosperms” (= flowering plants) and, by extension, of the so-called co-evolution between inflorescences and honey bees.
There are, however, other difficulties to explain features of the angiosperms. Most of them possess hermaphroditic flowers with male (stamens with anthers and pollen) as well as female (pistil with stigma and ovary below) parts. Some plants possess separate male and female flowers on the same individual plant and are termed “monoecious”, while species (roughly 10% of all flowering plants) in which separate male and female individual plants are the rule, are known as “dioecious”. If male and female structures are present in the same flower, pollination by insects is not a problem and even if there are separate male and female flowers on the same plant, it’s not such a big deal. If, however, there are considerable distances between male and female plants to overcome by pollinating insects, Nature needs to have come up with some solution. Unsurprisingly, temperate dioecious species are often wind- or water pollinated, but in tropical forests with little wind, bees, beetles, flies and moths, i.e. strong fliers, are more important. Floral rewards comprise pollen, nectar, mucilage, nutritious tissues, and resins, but about one third of the species offer no reward in the female flower morphs.
Some of the better known insect-pollinated, yet dioecious, species include, to name but a few, kiwi-fruit, wild grapes, spinach, asparagus, willow, polar, hemp, etc. For these plants to be pollinated it would make little sense if the pollinating insects would first visit female flowers and after that the pollen-bearing male flowers; the sequence needs to be male flowers first to pick up pollen followed by a visit to a female flower to deliver the pollen. To make sure of that, male inflorescences are usually somewhat larger and visually more attractive. Naïve, inexperienced foraging bees notice them easily and of course visit them to obtain sugary nectar. They pick up pollen from them and remember the flowers’ scent. And now comes the lure of the female flowers: although it’s possible to confuse them with the male flowers even though the former are smaller in size and less sightly, their smell is far more intense and alluring so that the bees hoping to find another pollen-rich nectar containing male flower visit them in error and deliver to them the pollen they had picked up earlier.
Differences between male and female plants can even affect not only the flowers but also the seeds as in the knotweed Rumex nivalus where male seeds are heavier and germinate before those of the female seeds or as in the spinach with its heavier male seeds and the White Campion (Silene latifolia) , in which male and female seeds differ with regard to dormancy and survival. In the South African Leucodendron xanthoconus male plants have more branches and smaller leaves than females and generally speaking in long-lived dioecious species (but in short-lived ones it’s the opposite) male individuals often exceed the females in vigour. Now, after all this, who would still believe that sex and reproduction in plants must be less interesting than reproduction in animals? I think this blog dispels that myth.
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