Gesneriads

These plants are mostly native to tropical regions, and are found on almost every continent. Some genera do prefer temperate or even semi-alpine climates like those found in the mountains of Asia, Europe and Central and South America. Some species, like Ramonda myconi, which grows in the Pyrenees, can even tolerate snow cover. Surprisingly, no member of this family appears to be native to North America.

Gesneriads are mainly found in shady tropical forests. But some Gesneriads do thrive in temperate climates in Chile, New Zealand, the Himalayas, Japan and southern Europe (the Balkan and Iberian peninsulas).

The vegetative organ

Habits

Geraniaceae, or Gesneriads, come in a wide range of forms, from tiny plants like Sinningia pusilla, which can grow for years in a thimble-sized pot, to small trees like Coronanthera grandis that can grow up to 15 metres tall.

Most species are herbaceous plants or suffrutescent (i.e. slightly woody at the base) perennials. Some are shrubs, such as these genera:

• Besleria
• Cyrtandra
• Coronanthera
• Paliavana
• Pentadenia
• Gesneria
• Vanhouttea

Most shrubby or woody stemme Gesneriads grow on the ground where exposure to sunlight is more or less direct. In the wild, Gesneriads prefer habitats such as glades, river banks and rock faces. The Napeanthus genus and several Gasteranthus and Sinningia species, however, grow better in moist sites, with indirect light. Genera with upright stems like Drymonia, Moussonia and Kohleria make excellent container plants and do very well on a window ledge.

The Geraniaceae family also includes some 600 epiphytic species in 28 genera. The Codonanthe and Episcia genera, for instance, produce creeping stems, a characteristic often found in epiphytic species, and grow on moss-covered rocks and tree branches.

Underground organs

Gesneriads have one of three types of underground organs:

• Fibrous (fasciculated) roots
• Tubers
• Scaly rhizomes

The last two organs store moisture and are used by many terrestrial Gesneriads to survive adverse conditions such as a dry season or drought.

Fibrous-rooted Gesneriads

Plants with fibrous roots do not have storage organs; they tend to grow constantly, without a marked dormancy period. This category contains a large number of cultivated species, such as Saintpaulia, Episcia, Chirita, Gesneria, Columnea and Streptocarpus. Because of their limited root system, fibrous-rooted Gesneriads, which include several epiphytic species, do very well as container plants.

Tuberous Gesneriads

The Gesneriad tuber is a perennial stem-like structure that expands with each growth cycle. The stems grow from the top part of the tuber. Tuberous Gesneriads generally go through a marked dormancy period, during which they lose their leaves for several months. This dormancy period is how the plants adapt to dry spells in their native habitat. One of the most widely known tuberous Gesneriads is the common gloxinia (Sinningia speciosa). Some members of the Nautilocalyx and Chrysothemis genera are also tuberous.

Scaly rhizome

Gesneriads Scaly rhizomes are underground stems with leaves modified into closely packed scales. They are usually small and have very little storage. Nonetheless, all rhizomatous Gesneriads go through a dormancy period in their native habitat or in appropriate growing conditions. Scaly rhizome Gesneriads belong to the Achimenes, Gloxinia and Kohleria genera, among others.

Foliage

Typically lance- or heart-shaped or oval, the Gesneriad leaf blade is always simple with an entire or toothed margin. Gesneriads generally have an opposite (decussate) phyllotaxis (leaf arrangement); some species have a whorled arrangement, while there are rare cases with an alternate pattern. Some genera – such as Codonanthopsis, Dalbergaria and Monopyle – include species in which anisophylly (unequal size of the two leaves of a leaf pair) occurs. Gesneriad species growing in tropical forests may have developed anisophylly as a way of adapting their photosynthetic organs to gain better access to available light. The resulting arrangement, where none of the leaves overlap, allows climbing and creeping plants to maximize their exposure to sunshine. Plants with opposite or whorled leaf arrangements typically have upright or trailing stems, whereas anisophyllous plants have climbing or creeping stems with leaves arranged in two ranks.

The foliage of some Gesneriads can be quite striking. Several species have large red patches on the underside or tip of their leaves; some sport metallic hues, while others have variegated foliage. In some species, leaves even play a role in pollination. At anthesis, the leaves of many species of the subfamily Gesnerioideae produce brilliant red markings, which act as extrafloral cues to attract hummingbirds.

Natural vegetative propagation

Gesneriads rely on two types of natural, asexual reproduction.

The first type is vegetative regeneration, in which the meristem, present either at the growing tip of the stem or at the base of the node, increases the length of the stem or the plant. This includes the formation of adventitious roots (all Gesneriads) or of calli that become tubercles (certain Chrysothemis, Lembocarpus and Sinningia) and the production of stolons that emerge from axillary buds (the Alsobia and Episcia genera).

The second type is the production of plantlets on leaves that fall to the ground. The species of many genera reproduce in this manner, such as Achimenes, Gloxinia, and Nautilocalyx, with the best known being Saintpaulia and Streptocarpus.

The reproductive organ

Most Gesneriaceae, or Gesneriads, have bright, showy flowers, making them attractive indoor plants. In some species, not only the corolla but also the sepals and bracts are colourful. The flowers may be solid, striped, speckled or bi-coloured. They measure one to twenty centimetres across and come in a wide range of hues, from red, orange and yellow to blue, purple and even green.

If grown under the right conditions, certain species belonging to the Nemathanthus and Aeschynanthus genera produce flowers that can last up to two weeks. In contrast, other Gesneriads, such as Napeanthus, have flowers that bloom for less than one day, opening at dusk and fading the next morning.

The flowers are bisexual, that is, having both male and female reproductive organs, and typically have dorsiventral symmetry; a few will have radial symmetry.

The calyx consists of four to five sepal lobes that are either free to the base or fused, of equal or unequal size, with entire or dentate margins. The petals, usually five or sometimes four, form a tube that can be short (Saintpaulia and Bellonia) or very long (Solenophora), narrow or pouched. The lobes, located at the tip of the tube, are either pointed or curved, ruffled or fringed. The corolla may be covered with fine hairs in contrasting colours. In some species, there are glandular hairs that produce a viscous substance inside the calyx tube.

Although most Gesneriad flowers have four stamens, there are sometimes only two or as many five. The stamens are attached at the base to the petals and the anthers cohere near the flower opening. Dehiscence is by longitudinal slits or less frequently by an apical or basal pore. Some genera have staminodes. The nectar is produced by a nectary disk or five nectariferous glands located at the base of the ovary, between the carpels and the stamens. Some genera present rudimentary or non-functional nectaries or have no nectaries at all, such as the Napeanthus genus.

The ovary is superior or inferior, unilocular or rarely bilocular. Placentation is parietal. Gesneriaceae have traditionally been separated from Scrophulariaceae by having parietal rather than axile placentation. And yet it has long been known that each of these families includes representatives with one or the other type of placentation. According to Wiehler (1983), some authors have shown that there is no fundamental difference between the figworts’ axile placentation and Gesneriads’ parietal placentation. Nevertheless, the type of placentation remains, in 96% of cases, a good way to differentiate between these two families. The simple style usually becomes elongated as the stamens retract. The stigma is usually either bilobed or stomatomorphic (mouth-shaped).

Most neotropical Gesneriads have flowers in which the development of the male organs is diphase. The stamens usually reach maturity first. The opposite is observed in species that are pollinized by bats: G. auriculata, G. leucomalla, G. tomentosa, G. vermicosa and G. viridiflora.

Gesneriad flowers are considered to be evolved: specific flower shapes and colours appear to have developed as a result of co-evolution with pollinators such as hummingbirds, insects and bats, with specific pairings between plant and pollinator species. The evolutionary diversification of the family and speciation is closely linked with the mode of pollination. The following flower characters play a role in pollination:

• the type, length and position of inflorescences;
• the shape, size, colour and texture of the calyx and the corolla;
• the position of the ovary;
• the presence or absence of nectaries;
• the shape, size and position of the nectaries;
• the scent produced by the flower; the shape and position of the stigma;
• the length of the style;
• the shape and position of the anthers.

Moreover, as we saw earlier, even vegetative organs can play a role in pollination.

Wiehler (1983) surmised that 50% of neotropical Gesneriads are pollinated by hummingbirds. This is an extremely high percentage, even for a family of neotropical plants. According to Wiehler, the distribution centre of neotropical Gesnerioideae (Colombia and Ecuador) coincides with that of the bird family Trochilidae, a group of hummingbirds with nearly 320 species and 450 subspecies. Most other species of neotropical Gesneriads are pollinated by insects, particularly Euglossine (orchid) bees (30%). Species pollinated by bats have greenish flowers, such as Kohleria digitaliflora.

There is a strong correlation between epiphytism and pollination by birds. For instance, the tribe Episcieae contains 670 species of which 468 are epiphytes; of these, no less than 80% are pollinated by hummingbirds.

In those Gesnerioideae that are pollinated by hummingbirds (Woods and Woods, 1984), the corolla varies from dark shades of red, orange or dark yellow and is one of four shapes: nearly actinomorphic, forming a more or less straight tube (Trichantha and Kohleria species); zygomorphic with an elongated tube (Columnea); urceolate, with a tiny opening (some Alloplectus and Besleria species); tubular, of varying widths, with an opening surrounded by free lobes (Episcia and Drymonia species). The final shape is a recent adaptation to bird pollination.

The fruit of the Gesneriads is either a berry or a dry (tribe Gloxinieae) or fleshy capsule.

Seed dispersal varies considerably within the family. The genus Aeschynanthus, for instance, produces narrow capsules, some more than 30 centimetres long, whose countless hairy seeds are shaken out by the wind. The twisted fruit of Streptocarpus dries as it ripens; when ripe, the seedpod twists open to slowly release the seeds. The capsules of certain Gesneria species are bowl-shaped; when it rains, the seeds are washed away.

Seeds found in berries are usually larger and less numerous than those found in capsules. The berries’ thin semi-transparent envelope sometimes acts as a greenhouse, allowing the seeds to germinate before they are dispersed. A rather fascinating phenomenon occurs in Codonanthe crassifolia. After a downpour, the berries will occasionally split and the exposed seeds will be carried off by ants; this represents an adaptation in which ants play a role in seed dispersal, especially because the seeds look like ant eggs. Chrysothemis friedrichstholia is another example of seed dispersal by ants (Folsom, 1984).

The production of berries can be considered an adaptation for seed dispersal by birds.

The seeds are extremely tiny – similar in size to those of orchids. They germinate and grow as quickly do Petunia seeds. The seeds of some Gesneriads will keep for several years; no special storage conditions are required.

The botanical classification of Gesneriaceae, or Gesneriads

The pan-tropical Gesneriaceae family is divided into two major subfamilies: the neotropical Gesnerioideae and the paleotropical Cyrtandroideae.

According to Burtt (1963), Gesnerioideae has cotyledons that are equal in size and is found for the most part in the New World, the exceptions being members of the tribe Coronanthereae and the genus Fieldia.

The subfamily Cyrtandroideae has cotyledons that are unequal in size and is found exclusively in the Old World, the exception being the New World genus Rhynchoglossum.

It is worth noting that in his superb work on Gesneriaceae Wiehler (1983) raised the tribe Coronanthereae to the level of subfamily, noting that it has features that distinguish it from Gesnerioideae, including its geographical distribution and a nectary that is adnate to the ovary.

Burtt’s classification, accepted by the majority of contemporary botanists, marks a significant step towards our systematic understanding of this family and provides an impressive example of taxonomic analysis.

Prior to Burtt’s work, which was carried out largely over the last four decades, the prevailing classification was that of Fritsch, a late 19^th century botanist still referenced in many modern botanical treatises (Melchior, 1964). Fristch divided the two subfamilies according to the position of the ovary. According to his classification, Cyrtandroideae includes genera with superior ovaries, while Gesnerioideae consists of genera with an inferior or semi-inferior ovary. The difference between the two classifications is quite significant (see table below). For example, according to Fritsh, a genus like Columnea belongs to the Cyrtandroideae, whereas Burtt places it in the Gesnerioideae subfamily.

Current belief is that ovary position varies too much from one genus to another to allow for a natural classification of plants in this family. Burtt’s contribution was to show that the number of cotyledons and geographical distribution are closely related. Each subfamily, therefore, forms both a geographic and a taxonomic entity. It can be said, consequently, that Burtt’s classification is more natural than Fritsch’s, and is also an improvement, since it better represents Gesneriaceae evolutionary history.

Early and modern classifications of Gesneriaceae

According to: Syllabus der Pflanzenfamilien (Melchior, 1964)

According to: Burtt (1963) and Heywood (1978)

Text exerpted from an article by Denis Barabé that appeared in the Friends of the Montréal Botanical Garden newsletter.