Gesneriaceae:
Inflorescence Structure
 

 

The special nature of the gesneriaceous inflorescence has been recognized only recently. It is important to note that the main axis of the plant never ends in a terminal flower (it is “indeterminate”) and all (partial) inflorescences are lateral (= axillary = emerging from the axils of foliage leaves or bracts). These lateral inflorescences represent a special type of flowering axis or cyme, the “pair-flowered cyme (Weber 1973, 1982). By reduction, these may proceed to solitary flowers.

 



Diagrams of a pair-flowered cyme with six flowers; T = terminal flower of cyme unit, F = front flower  (Weber, 1975)

 

Occurrence of pair-flowered cymes

As far as is known, the pair-flowered cyme is found only in Gesneriaceae, a few genera of Scrophulariaceae (Penstemon, Tetranema, Russelia), Calceolariaceae (Calceolaria, Jovellana, Stemotria) and Sanango. In this cyme the terminal flower (T) of each cyme unit is associated with an additional flower in frontal position (front-flower, V). So each cyme unit seems to end in a flower pair. In general, the front-flower has no subtending bracteole, while the lateral flowers are usually subtended by bracteoles (small-sized prophylls of the successive cyme units).

 

Variation

There is considerable variation in branching and flower number within the pair-flowered cyme. Usually, the first (basal) branching is bilateral (“dichotomous”, but there are many cases in which branching is consistently unilateral (e.g. in all members of Epithematoid Gesneriaceae). The number of flowers (or flower pairs) within the cyme may be high (e.g. unifoliate species of Streptocarpus), moderate or low. In case of extreme reduction only the first flower (T1) remains. “Solitary” flowers occurring in the leaf axils of many neotropical gesneraids always means reduction of an original pair-flowered cyme to a single flower. If additionally the subtending leaf of the flower is reduced to a bract, the flowers of the whole inflorescence region aggregate to a raceme (e.g. Diastema, Smithiantha, Gloxinia). Occasionally, a spike is reached by extreme shortening of the pedicels (e.g. Sinningia allagophylla).

 

Special functions of bracteoles

The bracteoles within the pair-flowered cyme are usually small and inconspicuous. However, by enlargement and bright coloration they may take part in the attraction system for pollinators (e.g. Drymonia species). By the enlargement and fusion of the first bracteole pair, a conspicuous cupule embracing the flowers is formed in some species of Cyrtandra (e.g. C. cupulata, C. burbidgei).

 

Increase of pair-flowered cymes per leaf axil

By serial repetition, often more than one cyme (or solitary flower) can be present in a leaf axil. Particulary remarkable are the “crested” inflorescences of some species of Chirita sect. Microchirita: here the pair-flowered cymes are reduced to flower pairs (T1, V1) and such pairs are produced in considerable number in a line on the petiole of the subtending leaf (Weber 1975b).

 

Previous Section

Next Section