Development of the axillary bud complex in Echinocystis lobata (Cucurbitaceae): interpreting the cucurbitaceous tendril.

In the Cucurbitaceae, the tendrils, coiling organs used for climbing and mechanical support, are part of an axillary bud complex (ABC). Although the morphological nature of tendrils and the branching pattern of the ABC in the Cucurbitaceae have been much studied, their homology remains unresolved, with hypothesized candidates being the leaf, flower, stem, or stem-leaf combination. We used Echinocystis lobata as a model to study the early ontogeny of the ABC with epi-illumination microscopy and serial resin sections. The ABC produces four structures (proximal to distal, relative to the subtending leaf) as the result of two successive subdivisions: an inflorescence of staminate flowers, a solitary pistillate flower, a lateral bud, and a tendril. The first separates the tendril primordium from the continuation of the ABC, and the second separates the staminate inflorescence and the ABC. The pistillate flower apparently forms between the staminate inflorescence and the lateral bud. Because there is no subtending leaf during these subdivisions and the first lateral appendages in the resulting primordia arise in the same plane, we conclude that the tendril and other organs formed by the ABC are lateral branches of equal morphological value. This study is the basis for continuing comparative and functional morphological studies.