Radiation and Floral Adaptation in Gladiolus

The brilliant scarlet flowers of Gladiolus saundersii, native to the Drakensberg of South Africa and Lesotho, flowers in late summer when its pollinator, the large butterfly Aeropetes is on the wing. Author’s photographs

The brilliant scarlet flowers of Gladiolus saundersii, native to the Drakensberg of South Africa and Lesotho, flowers in late summer when its pollinator, the large butterfly Aeropetes is on the wing. Author’s photographs

Gladioli have been cultivated for longer than two hundred years and there can be very little doubt that the genus has been Southern Africa’s most important contribution to the world of horticulture…

Niel Du Plessis and Graham Duncan, Bulbous Plants of Southern Africa

Plant breeding and selection have done the genus Gladiolus an extraordinary disservice. Those large-flowered, long-stemmed, and admittedly handsome plants are nothing at all like any wild Gladiolus. They are the result of over 150 years of hybridizing of just a handful of wild species and then selecting generations of hybrid progeny. Gladiolus is a member of the iris family (Iridaceae), and although Gladiolus is not particularly closely related to Iris, the genera share the distinctive sword-shaped leaf and a flower with six tepals and three stamens. Gladiolus is almost exclusively African; just a few species occur in Europe and the Middle East. Its real center of diversity is southern Africa, that crucible of plant evolution and radiation.

For many years Gladiolus has been regarded as a terribly complex genus and a nightmare to those trying to identify wild species. Not so any longer. As a result of studies over the past twelve years, we have collaborated on the compilation of botanical monographs of the species in tropical Africa and more recently of those in southern Africa where all but ten of the species are native. We can now write confidently about this remarkable genus, about which there has been no authoritative information published until recently. And Gladiolus is remarkable in many ways. With over 250 species, it is one of the world’s larger plant genera, and, within the iris family, it takes second place only to Iris, which may have 300 species. But numbers alone, impressive as they are, give no hint of the diversity of plant and flower form and the fascinating adaptive radiation of the genus. Plants range from dwarf species of the semi-deserts of western southern Africa to some giants up to more than six feet (two meters) tall in well watered parts of eastern southern Africa. Then there is the floral diversity, so extensive that, for many years, some species of Gladiolus were classified in several other genera.

Flowering in late summer and fall, the dull, speckled flowers of Gladiolus emiliae are well camouflaged in dry grass, but show up surprisingly well at sunset when their intense, sweet odor is strongest. They are visited at night be a range of large, long-tongued moths, their legitimate pollinators

Flowering in late summer and fall, the dull, speckled flowers of Gladiolus emiliae are well camouflaged in dry grass, but show up surprisingly well at sunset when their intense, sweet odor is strongest. They are visited at night be a range of large, long-tongued moths, their legitimate pollinators

The Impact of Pollinators

Floral diversity, it turns out, is the key to understanding much of the biology and natural history of gladiolus. Research on the pollination biology of gladiolus, conducted over several years with funding from the National Geographic Society and the National Science Foundation, has allowed us to understand the basis for much of the variation. That old biological rule that form follows function was never more evident. The genus has what are generally regarded as gullet type flowers; that is, they have tepals arranged around a throat leading to a tubular base. The lower tepals often have contrasting markings that serve as nectar guides to help insect visitors orient themselves correctly as they approach a flower seeking some food reward, this being the reason most insects visit flowers. In most gladiolus the reward is sugar-rich nectar contained within the floral tube. The reward is provided so that an appropriate visitor will be passively dusted with pollen as it forages. On subsequent visits to other flowers, it will inadvertently transfer pollen to their stigmas, thus achieving cross-pollination and ensuring the production of seed—and a new generation of plants. Without the activity of pollinating insects or other animals, most plants, including gladioluses, will not produce seed.

Species of Gladiolus, we have learned, are remarkably specialized in the range of insects and birds that pollinate them, and a great deal of the floral diversity in the genus is precisely correlated with a particular pollinator. The length and shape of the floral tube are vital factors in determining the pollinator. Bees have tongues seldom exceeding one-half to three-fifths of an inch (twelve to fifteen millimeters); they readily feed from gladiolus flowers with tubes of corresponding length. Species of Gladiolus come with tubes of a wide range of lengths, from short to extremely long. Tubes longer than four-fifths of an inch (twenty millimeters) signal an adaptation to pollinators other than bees. If the flowers are red, as they are in many species, the most likely pollinators are sunbirds. This is because many potential pollinating insects see colors in the red range poorly, or do not associate these colors with a food source, and thus do not visit red flowers.

With pale cream flowers and red markings, the long-tubed flowers of Gladiolus engysiphon are typical of Gladiolus species pollinated by long-proboscid flies

With pale cream flowers and red markings, the long-tubed flowers of Gladiolus engysiphon are typical of Gladiolus species pollinated by long-proboscid flies

Long-tubed Flowers

This is not entirely true in southern Africa, for the large satyrid butterfly (Aeropetes) has an unusual affinity for red flowers and, in fact, is the sole pollinator of a suite of red-flowered plants. Among these are several species of Gladiolus that flower only in mid- to late summer when the butterfly is on the wing. It is sometimes difficult to distinguish bird and butterfly flowers but not in gladiolus: bird flowers in the genus have fairly wide tubes, to accommodate the large bill, and lack nectar guides, whereas butterfly flowers have fairly narrow tubes, butterflies having slender probosces. Butterfly flowers also commonly have whitish splashes on the tepals, perhaps for orienting the butterfly. Thus, looking at the brilliant scarlet flowers of the New Year gladiolus (G. cardinalis) the biologist may first assume bird pollination, but the white markings and the narrow tube, as well as the drooping stems, seem to exclude relatively heavy sunbirds, which must perch while feeding. And indeed, birds seldom visit the flowers of this species, for they are too awkward to manipulate and yield too small a reward for a bird. The Aeropetes butterfly is, however, its legitimate pollinator, and it can often be seen hovering about the flowers of this species or perching on the tepals as it feeds.

Other long-tubed flowers of Gladiolus are cream to pink with red markings and—unusual for the genus—lack any scent. A narrow tube prevents bees from reaching the nectar, and the flowers are not attractive to butterflies; instead, they are adapted for a specialized pollinator, a long-proboscid fly. Long-tubed pink flowers are particularly common in southern Africa, largely because this kind of flower seems to attract this specific and effective pollinator. These flies may have probosces up to two inches (fifty millimeters) or, in rare cases, even four inches (100 millimeters) long, and the flowers they feed from have correspondingly long tubes. The Cape species, G. carneus, locally called the painted lady, and G. virgatus, are just such species. What a treat it is to actually encounter one of the flies that are their exclusive pollinator. They belong in either of two fly families, the horseflies (Tabanidae) and tangle-veined flies (Nemestrinidae), that occur all over the world. Only in southern Africa, however, are some of the flies in these two families adapted to feed from long-tubed flowers. The adult flies depend entirely on nectar for their nutrition, although female tabanids must also feed on blood (as most field biologists can painfully attest to) before they can lay eggs.

Moths are also common pollinators, and gladioluses have their share of moth-adapted flowers. Typically, moth flowers are white or cream and are strongly scented, this combination being most effective in attracting night-flying insects. While other gladiolus flowers usually partly close at night, moth flowers are at their best after sunset when the petals stiffen up and their scent is released or intensified. Some gladioluses have perfected a trick that is rare in the plant kingdom: they change color at night. Gladiolus liliaceus flowers have dull, earth-colored tepals during the day but, at sunset, change to a light purple that shows up brightly as the daylight fades. The dull coloration is presumably for camouflage, making the flowers less visible to either herbivores or nectar thieves during daylight hours. Sphinx moths, among the most important of floral pollinators, have long probosces, and the gladiolus species that have moth-pollinated flowers have correspondingly long floral tubes. In tropical Africa, sphinx moth pollination is thus assumed for G. murielae, a species once classified in the genus Acidanthera, kept separate from Gladiolus because of its tubular flowers, which in this species may be one-half to three-fifths of an inch (twelve to fifteen centimeters) long.

An Anthophora bee visiting the bright red, flag flowers of Gladiolus alatus and about to be dusted with pollen on its dorsal thorax by the dropping anthers

An Anthophora bee visiting the bright red, flag flowers of Gladiolus alatus and about to be dusted with pollen on its dorsal thorax by the dropping anthers

The Bee Connection

What a contrast, then, from such huge tubular flowers to the star-like flowers of Gladiolus stellatus, native to the southern Cape region of South Africa. This species and a few others like it have shifted their pollination system from one of passive pollen transfer—using insects that visit flowers seeking nectar and only incidentally transport pollen on their bodies—to an active pollination system where pollen itself is the reward. Thus, G. stellatus boldly displays its pollen bearing anthers, and the dull colored, rather small flowers depend on their heady scent and visible reward to attract female bees, which collect pollen to provision nests for their young.

Bees are the world’s most important pollinating insects, and bee flowers are the most diverse. This is also true of gladiolus. The star flowers of bee-pollinated gladioluses come in almost every color of the rainbow, even red, orange, green, and brown. And bee-pollinated gladioluses usually have the strongest scents. Scent is not a feature usually associated with the garden hybrid gladiolus; it was lost when breeders selected for flowers with bright colors and large size. Sadly, some of the most finely scented species of Gladiolus have never even been used in gladiolus breeding.

Why should bee flowers have become so diverse when the same few bee species are known to pollinate the same range of Gladiolus species? We have often wondered if it is because of the condensed flowering season, especially in the southern African winter-rainfall zone where, in fact, the genus is most diverse. After a cold wet winter, the flora bursts into flower in August and September and there may just not be enough bees around to pollinate all the flowers. If that is the case, then species with the most striking flowers or those offering the most distinctive shapes, colors, or scents (combined with an appropriate reward) would be the ones most likely to be visited. Gladiolus seems to have successfully exploited this strategy; fully fifty-five percent of the species in the genus are pollinated by bees—and nearly all of them by long-tongued bees seeking nectar.

The specialized flowers of Gladiolus stellatus are quite unlike typical flowers in the genus; they have symmetrically disposed tepals and stamens, and anthers prominently displayed. An extraordinary strong, sweet scent compensates for the rather dull color of the perianth

The specialized flowers of Gladiolus stellatus are quite unlike typical flowers in the genus; they have symmetrically disposed tepals and stamens, and anthers prominently displayed. An extraordinary strong, sweet scent compensates for the rather dull color of the perianth

Diversification in the genus seems, then, to have proceeded in one of two directions: either flowers become more striking and distinctive since they must keep attracting bees in order to reproduce and survive, or they shift away from bee pollination to a more specialized system. In the latter option, floral diversity ceases to be important and flowers must conform to the pattern that each specialist pollinator seems instinctively drawn to. Thus, bee flowers are highly diverse, as can readily be seen comparing the scarlet flowers of G. alatus with the purple and yellow flowers of G. venustus with those of G. stellatus. In contrast, long-proboscid fly flowers, or moth flowers, or bird or satyrid butterfly flowers are each channeled into more stereotyped patterns.

In Gladiolus, at least, the floral diversity in the genus can be partly understood in terms of adaptive radiation for pollination systems. In other words, the apparently bewildering range of species in the genus can be rationalized as populations of plants with distinctive flowers, each adapted closely for a particular pollinator.