Developmental Aspects of the Pair-Flowered Cyme of Gesneriaceae

Developmental aspects of the pair-flowered cyme of Gesneriaceae are summarized. As emerges from a study of the cyme development of Sinningia Bulbosa, the front-flowers are true axillary flowers. The ɣ-bracteoles are initiated later than the prophylls and the same applies to their axillary structures. Because the primordia of the front-flowers do not produce prophylls and proceed directly to the formation of the floral organs, they are able to develop more rapidly than the lateral flowers and open earlier than these; the acropetal sequence of initiation is thus blurred and appears as a basipetal one (which pattern would contradict a derivation of the pair-flowered cyme from a paniculate branching system). By ontogenetic abbreviation the development of many-flowered cymes (apparently always in the form of ebracteolate cincinni) may switch to a pseudo-monopodial pattern. Examples from paleo- and neotropical Gesneriaceae are cited Possible developmental mechanisms of bracteole displacement are briefly discussed.