Secondary and Tertiary Pulvini in the Unifoliate Leaf of Cercis canadensis L. (Fabaceae) with Comparison to Bauhinia purpurea L

• Published in: International journal of plant sciences Vol. 161; no. 4; pp. 583 - 597
• Main Author: Owens, Shirley A.
• Format: Journal Article
• Language: English
• Published: Chicago The University of Chicago Press 01-07-2000; University of Chicago, acting through its Press
• Subjects: Botany; Flowers & plants; pulvinus; Cercis canadensis; bilobate leaf; Bauhinia purpurea; unifoliate leaf; leaf movement
• ISSN: 1058-5893; 1537-5315
• DOI: 10.1086/314283

There are three leaf types in the Cercideae tribe (Caesalpinioideae,
Fabaceae): unifoliate, bilobate, and bifoliate. By the fusion hypothesis,
the ancestral leaf type within the Cercideae was bifoliate, and the
unifoliate leaf was derived from the fusion of the bifoliate leaflets. In
the opposing splitter hypothesis, the unifoliate leaf was the ancestral
leaf type within the Cercideae, and the bifoliolate leaf was derived from
the splitting of the unifoliate leaf into two separate leaflets. Palmate
venation in leaflets is problematic for the fusion hypothesis. Watari
proposed derivation from a palmately compound leaf with many leaflets and
interpreted the primary vein as homologous to a single midrib in a leaflet.
The structure and function of the upper pulvinus in the unifoliate leaf of
Cercis canadensis L. were examined to test the two hypotheses.
Microscopic observations and measurement of leaf movement over a
12-h period indicate that the upper pulvinus in the unifoliate leaf
is a compound structure consisting of an apical common joint composed of
three secondary pulvini and two lateral laminar joints separated by a
laminar joint associated with the midrib. A lateral laminar joint
consists of three tertiary pulvini, one at the base of each primary vein,
and the midrib is formed from a rachilla and two or more pinnules.
Structure comparisons were made between the upper pulvinus of a unifoliate
leaf of C. canadensis and a bilobate leaf of Bauhinia
purpurea. The secondary and tertiary nature of the upper pulvinus
indicates that the unifoliate leaf in Cercis was derived from
an extinct ancestor with bipinnately compound leaves. Results add support
for Watari's interpretation of the primary vein and for the splitter
hypothesis.