Shirley A. Owens

Degradation of the upper pulvinus in modern and fossil leaves of Cercis (Fabaceae)

Identification of fossil leaf impressions as Cercis has been questioned based upon the presence or absence of a pulvinus at the base of the lamina (upper pulvinus). In the present study, leaves of Cercis canadensis were examined before and after abscission to explore the degradation processes that could occur prior to fossilization, and the North American record for fossil foliage of Cercis was revised accordingly. Results for C. canadensis indicate that: (1) the pulvinus consists largely of tissues with nonlignified cells (a wide cortex, a nonlignified fiber sheath, phloem, and pith) that degrade rapidly after leaf abscission, (2) the lignified xylem tissue that remains in the pulvinus after degradation is in brittle strands, (3) the pulvinus degrades at a faster rate than the lamina or the petiole, and (4) the degraded pulvinus cushion leaves a semicircular pattern on the lamina. From examination of fossils as well as extant species, we: (1) demonstrated that in fossils, the upper pulvinus can show a greater degree of degradation than the adjoining petiole or lamina tissue, suggesting the degradation of upper pulvinus tissue is similar in modern vs. fossil specimens, (2) defined numerous other laminar characters that can be used in conjunction with, or in the absence of, an upper pulvinus to confirm the presence of Cercis in the fossil record, and (3) showed from those criteria that the earliest known North American fossil leaf record for Cercis, from a specimen newly reported in the present study, is from the middle Miocene Succor Creek flora of Oregon.

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

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.

EXPERIMENTALLY INDUCED CHANGES IN BUD FATES IN CERCIS CANADENSIS L. (FABACEAE)

Stems of Cercis canadensis L. were pruned to three differently aged segments at three different times during the summer of 1992 to determine the effects of treatment on bud fates (vegetative or reproductive). In addition, the development of vegetative and reproductive buds was microscopically examined on untreated stems. In C. canadensis, multiple axillary buds are produced in a linear series at each node. The distal-most buds on the distal-most nodes become vegetative, and the remaining buds sequentially mature into reproductive buds over a 1-5-yr period. Microscopic buds elongate and become macroscopic in the summer prior to maturing the next spring. The number of inflorescences that elongated in 1992 was not affected by the pruning treatment, but of those, the number of inflorescences that aborted in 1993 increased with the number of nodes pruned. Deeper pruning thus reduced the number of mature inflorescences per node. Some buds that would have normally developed into reproductive shoots became vegetative shoots after stems were pruned. Microscopic observations of buds from nontreated stems indicated that in all of the buds, initial leaf primordia had an orthodistichous phyllotaxy, but in those buds that become inflorescences, the phyllotaxy changed to helical during floral initiation. Although a reversal in eventual bud fate occurred in pruned stems, phyllotaxy indicated that the buds are initiated in the vegetative state; therefore, a reversion from the floral to the vegetative state did not occur. Intermediate shoots produced on some experimental stems developed four foliage leaves instead of four bracteose leaves, but the flowers on the inflorescence appeared normal.