Gesneriaceae:
Pollination (part 2)
 

 

Hummingbird flowers

Hummingbird flowers are typically red, orange or bright yellow. In the ornithophilous taxa of Episcieae and Sinningieae the corolla shape can be roughly classified into four types (Wiehler 1983):

  1. "tubular": tube cylindrical, straight or slightly curved, limb subregular (e.g., species of Achimenes, Moussonia, Kohleria, Sinningia, in the majority of species within Columnea sects. Collandra, Pentadenia, Stygnanthe and Ortholoma);

  1. "columneoid": tube narrow at base, expanding apically, limb strongly zygomorphic, divided into a prominent galea (consisting of the two enlarged dorsal lobes and two triangular lateral lobes) and a long and narrow, usually deflexed ventral lobe (e.g. Columnea sect. Columnea);

  1. "hypocyrtoid" (urceolate): tube ± strongly inflated (pouched), mouth much constricted (“target flowers“; e.g., Pearcea hypocyrtiflora, many species of Gasteranthus and Nematanthus); the pouch obviously serves to enhance flower visibility, while the tiny entrance ensures that the bird bill touches the anthers and stigma;

  1. "converted melitto-/euglossine flower": corolla shape as in bee flowers, but coloration deep orange or red (instead of white or light yellow). This type apparently represents a recent switch from melitto-/euglossophily to ornithophily, (e.g., species of Episcia, Nautilocalyx, Drymonia and Kohleria).

An additional "converted" type, combining characters of (a) and (b) (flowers tubular, limb strongly bilabiate the large upper lip projecting forwards), is found in Pheidonocarpa and many species of Sinningia and Gesneria.

 

Extrafloral cues in hummingbird-pollinated species

Apart from flower characters also extrafloral cues may play a significant role in the attraction of hummingbirds: large, coloured bracteoles in the inflorescences (e.g., Drymonia spp.) or red blotches, margins or translucent windows on the foliage leaves (Columnea sect. Collandra) The flowers are, in contrast, often rather inconspicuous and hidden in the foliage. As has been observed in Columnea florida, young and inexperienced hummingbirds (Phaethornis longuemareus and Heliodoxa jacula) first aim at the translucent red "windowpanes" on the leaves and then spend several seconds inspecting various portions of the plant before finding the flowers. Experienced birds hover briefly above the red spot and then dip quickly under the leaves and approach directly the axillary flowers (Jones and Rich 1972).

  

Pollination by bats

Bat pollination is only known (or presumed) in neotropical Gesneriaceae and apparently has evolved independently within several genera (Capanea, Drymonia, Kohleria, Gesneria, Rhytidophyllum, Sinningia and Paliavana). The flowers are short- and broad-tubed or distinctly campanulate, the limb is ± zygomorphic, coloration is yellowish or greenish with brown or dirty violet spots; nectar is produced in abundance. The chiropterophilous pollination syndrome of some neotropical taxa has been described in detail by Vogel (1958, 1968, 1969a,b) and Skog (1976). Pollination of Sinningia brasiliensis and Paliavana prasinata by Phyllostomidae-Glossophaginae has been recently documented by San Martin-San Martin-Gajardo and Sazima (2002a,b). Both species exhibit a typical chiropterophilous pollination syndrome that includes nocturnal anthesis, unpleasant scent and increase of nectar secretion in the night.  The photo to the right is by O.  von Helversen.

 

Floral radiation within genera

Some genera, especially of the neotropical Gesneriaceae, exhibit a wide range of pollination syndromes, e.g., Sinningia (bee-, euglossine-, butterfly-, moth-, hummingbird- and bat-syndrome), Achimenes (bee-, euglossine-, butterfly-, hummingbird-syndrome), Gloxinia s.l. (bee-, euglossine-, andro-euglossine-, hummingbird-syndrome), Kohleria (bee-, euglossine, hummingbird, bat-syndrome), Gesneria (hummingbird-, bat-syndrome).

In the Sinningieae nectar sugar composition in relation to pollination syndromes was studied in detail.  In the hummingbird and bee flowers (representing 95% of the species) nectar was found to be sucrose-dominant, with an average sugar concentration of 23.9 ± 10,6% and 28,7 ± 10.6%, respectively. The nectar of the sphingophilous and chiropterophilous flowers differed significantly in the hexose (fructose + glucose) dominance, with a low sugar concentration (7.1 ± 3.4%) in bat flowers (Perret et al. 2001).

 

Self-pollination

Self-pollination (facultative or obligatory) is known or suspected for a rather small number of Gesneriaceae (e.g., species of Epithema, Rhynchoglossum, Monophyllaea). Their flowers are mostly small and inconspicuous, often produced in large numbers, and fruit set is almost 100%. In a few cases the flowers do not open at all and fertilisation is within the flower bud (e.g., Streptocarpus nobilis, Codonanthopsis dissimulata). 

 

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