Sexy Angiosperm Plants

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. 

© Dr V.B. Meyer-Rochow and, 2022. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

Taking a Stroll in the Park (a Mossy Park that is) …

But you’ve got to be tiny

I remember that when I got back to Scott Base from a stroll up Observation Hill and showed our botanist Keith Thompson the photo of a cluster of moss plants he exclaimed: “Wow, you found a forest in Antarctica!” And now I wish to get a little deeper into the topic of a moss being called a forest.

Mosses are Bryophytes, usually rather small plants with thin leaves of just a single cell layer and no proper root system. Even the tallest moss, Dawsonia from New Zealand, with a spore-bearing stalk up to 50 cm tall, lacks like all of the other smaller Bryophyte  species, the typical vascular system and flowers of the higher plants. These tough little plants, with an ancestry that goes back 400 million years or more, were planet Earth’s terrestrial pioneers. With the exception of some freshwater species like Fontinalis antipyretica (there are no mosses in the sea) and those of swamps and bogs like Sphagnum, whose dead layers form the peat, Bryophytes are known to grow on almost anything that at least occasionally becomes soaked in water:  stones, tree trunks, bottles, roofs, monuments, and even other living plants and animals. The weevil Symbiopholus of Papua Niugini may support the growth of moss on its back and sloths, too, can have mosses in their fur. What is sometimes referred to as “moss caterpillars”, however, are not caterpillars with moss on their backs, but the larvae of nymphalid butterflies that possess moss-resembling protuberances on their bodies.

As a student in the Botany course many, many years ago, the plants I loved most were the true mosses (not including liverworts). The stage in their life cycle that has leaves and we recognize as a moss, has sexual organs and represents the gametophytes. Yes, mosses possess so-called antheridia  (which produce bi-flagellated sperm) and archegonia (which produce the egg cells). The reproductive organs, depending on the species, may be on the same or two different plants. The plants and thus their sperm and egg cells are haploid. To produce a fertilized egg, a sperm needs to reach archegonia where it can find an egg cell. For that ‘journey’ the sperm needs to swim and must wait until there is sufficient moisture. Once an egg cell has been fertilized, a new structure (attached to and growing out of the moss’ leafy gametophyte) develops and becomes noticeable as the thin and sometimes several cm tall, unbranched sporophyte. This structure possesses diploid cells and develops at its tip a spore-containing capsule. The spore-producing cells undergo meiosis, so that the spores are all haploid, some with male and some with female traits. When the spore capsule opens to release the spores, the wind carries them to various places or in the rare cases of species that grow on carrion or dung little flies may carry away spores. Should the spores land in a suitable spot, they grow into a thin threadlike protonema, which resembles a green alga, before changing into the more familiar moss with its little stems and green leaves. Being able to soak up water (but also surviving months without it), these moth gametophytes form a habitat for a multitude of invertebrates. 

Most famous of the latter are the cute tardigrades, champion survivors, just like the ever present rotifers and some tiny roundworms. Using a magnifying glass and even with the naked eye one would almost certainly encounter springtails, various mites, tiny eyeless arthropods known as Protura and Diplura, and probably very small flies, minute beetles, book lice (Psocoptera) and thrips. If one is really lucky, one may come across some pseudo-scorpion, a top predator in this micro-world. Easily recognizable as relatives of centipedes, but much smaller and thinner, are the multilegged and pale Symphyla and Pauropoda. Yellow Geophilus centipedes, ants and tiny micryphantid spiders would be the giants in this ‘forest’ and visible with the naked eye, but to spot the many ciliates and bacteria that are present, you’d need a microscope. Organisms like larval craneflies, caterpillars of micropterygiid mini-moths, and stigmaeid mites, feed on the mosses’ green parts and some nematodes are even known to induce the formation of tiny moss plant galls. Unfortunately, such a variety of different organisms would not have been present in my Antarctic “moss forest”, but take a clump of green moss sometime and go exploring:  take a walk on the wild side (of the mini world).     

© Dr V.B. Meyer-Rochow and, 2022. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

Erroneous Blooming in Autumn or Winter

When Nature Makes Mistakes 

Spring plants flowering in autumn; dragonflies laying their eggs on the tarmac of an asphalt road (which on account of its reflected polarized light they mistake for a freshwater stream); bulls used for breeding and artificial insemination ejaculating into a tube held by a collector inside a dummy model cow that the sex-starved bull mounts; the greedy toad consuming a Ramphotyphlops braminus “blind snake” that after having survived the passage through the toad’s digestive system wriggles out of the toad’s anus alive; the false or so-called “pseudo-pregnancies” relatively common in feral and wild dogs: are they (and many others like them) not examples of the fact that mistakes abound in Nature and Nature isn’t perfect? I suppose not to be 100% perfect is the difference to “cold” mathematics and, thus, the charm of Nature (and perhaps of some of our loving partners, too).

But what about plants that usually flower in spring, but suddenly produce some blossoms in autumn or winter? I’ve seen that as a child with plums and other trees and yet again here in South Korea with plants that were in full bloom during spring, but now in late autumn or early winter sport an isolated and lonely flower on their branches. Insects that could pollinate such flowers are mostly no longer present and it would be too late for a fruit to ripen let alone develop. What goes on and how can Nature allow this to happen; something which must be a waste in view of the plant’s survival strategy for winter. Actually, many plants that flower in early spring, and this includes plum and apple trees, already form their buds during the preceding summer. When the next spring arrives, the buds open. It’s the period between the formation of the buds and when they open, which is the key to understand why mistakes occur. It seems that weather conditions are erroneously interpreted by plants that allow their buds to open prematurely in autumn so that they can have a ‘head start’.

Winter dormancy for the vast majority of plants represents a period of stress: short days with little sunshine, reduced precipitation and then often only in the form a snow, low temperatures and low humidity. Under such conditions, it is best for plants to ‘slow down’ and get ready for better times to come. For some very ‘impatient’ plants such seemingly better times appear to have come when after a short spell of very dry and cold nights, perhaps in combination with a period of dry weather before the cold spell, suddenly temperatures rise again for a few days, the sun shines and moisture enters the soil. It gives these most impatient plants the signal to “go for it” and quickly open their flowers before the competition gets ready. However, it’s a stark mistake and the only beneficiary may be some of the few late-flying and cold-hardy insects still around. But why are there a few insects at all?

The reason is not the presence of the occasional autumn flower of a species that would normally erupt in inflorescences during spring; it is because there is a small number of plant species that always and naturally produce their flowers in autumn or early winter. Some of the plants that flower in late autumn, mid winter or early spring include the Mahonia tree, snowdrops and bluebells as well as some pansies, Hellebore and Grevillea species. For them the advantage is that they can have the very few pollinators still (or already) around, such as bumble bees, some moths and certain flies, to themselves. It’s similar to one take-away shop still open at night when all the others are already closed. The few customers still around at night will all shop at those very few places that are still open at that time. Are plants with wrong flowers in autumn then bad ‘thinkers’? Well, “errare  humanum est”  (-not only, it seems).

© Dr V.B. Meyer-Rochow and, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.