Research Article |
Corresponding author: Harold Robinson ( robinsoh@si.edu ) Academic editor: Peter de Lange
© 2016 Harold Robinson, Stanley Yankowski.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Robinson H, Yankowski S (2016) The Taxonomic Significance of ducts in the corolla lobes of Vernonia (Vernonieae: Asteraceae). PhytoKeys 58: 1-7. https://doi.org/10.3897/phytokeys.58.7009
|
The multiple longitudinal ducts in the corolla lobes found in the closely related genera Vernonia, Vernonanthura and Trepadonia are microscopically studied and illustrated. The lack of such ducts in the two South American species that have until now been retained in Vernonia indicates that they should probably be excluded from the genus.
Ducts, corolla lobes, Vernonia , Vernonanthura , Trepadonia
The generic concept of the genus Vernonia was simple and uninsightful for over 150 years from the time of it’s inception in Schreber (Gen. 2: 541. 1791, nom. cons.), with the eastern North American type Serratula noveboracensis L., type cons. Vernonia was the core genus of the tribe Vernonieae, having the combination of capillary pappus and non-liguliform florets that were common to all core genera of the Asteraceae during that time. Some segregates were named, such as Baccharoides Moench. and Gymnanthemum Cass., in Africa, Eremosis Gleason in Mesoamerica and Critoniopsis Sch.Bip. in South America, but these were not generally recognized at the generic level.
The concept of Vernonia began to change in the last 30 years as is partially summarized in the treatments by Robinson for the Paleotropical (
A unifying anatomical structure of the three related genera in the typical element of the tribe Vernonieae is found in the study of the corolla lobes of the group. The corolla lobes of Vernonia are filed with multiple parallel ducts containing resin or oils of some type (Fig.
A–D: A, B, Vernonia noveboracensis cross-sections of corolla lobes, outer surface upward C longitudinal section of corolla lobe showing multiple ducts D corolla lobe from specimen in preservative showing only poorly defined partitions between ducts E V. angustifolia, corolla lobe from freshly collected specimen showing fresh resin in ducts F Vernonanthura brasiliana, corolla lobe from dried herbarium specimen showing solidified contents of ducts with walls over-lying cell layer G Vernonia echioides, part of corolla lobe from dried herbarium specimen showing some short-stalked capitate glands H Vernonia incana, part of corolla lobe from dried herbarium specimen.
All initial observations of the ducts were in material mounted in Hoyer’s Solution, and no contents of the ducts were observed. It was when some material was observed microscopically in water mounts that resin or mucilage was seen in the ducts. Given the potential taxonomic importance of the ducts, a first determined effort is made here to provide a proper illustration of the structure.
Material examined was from dried specimens in the US National Herbarium (
For purposes of microtome sectioning, living material of Vernonia noveboracensis was field collected and fixed in formal-aceto-alcohol (
For views of unsectioned material, corolla lobes were taken from fixed material and from dried herbarium specimens and one freshly collected specimen for whole mount observation.
Images (photomicrographs) were captured in bright field or Nomarski (DIC) using a Zeiss Standard 16WL microscope, a Zeiss Universal Research microscope or a Zeiss Axiophot equipped with a Retiga 1300i digital camera (Q Imaging Corp.) and an image acquisition and processing system capable of live tiling and live EDF (extended depth of field) by MediaCybernetics/ImageProPlus 7.0.
The material from dried herbarium specimens was remoistened in water and mounted on microscope slides in water to preserve any resin or mucilage that might be in the ducts.
There are two ways of showing the ducts in the corolla lobes of Vernonia. One is by sectioning the material, the other is by whole mounts of the corolla lobes placed under the microscope. Both methods have been used here, and both methods have unmistakably demonstrated the ducts.
Cross-sections of the corolla lobes of Vernonia noveboracensis clearly show the series of seven to twelve ducts in a lobe (Fig.
The second method of viewing the ducts is the one that showed the structure in the paper describing Trepadonia (
Figures
The survey of the whole tribe for presence of multiple ducts has not been rigorous, but certainly on a par with the observations that led to the initial discovery of the ducts in Trepadonia, Vernonanthura and Vernonia (
The nature of the chemical contents of the ducts remains unknown. The types of chemicals known from the Vernonieae include epoxy resins, but these have been found most notably in genera like Stokesia L’Hér. (
It seemed particularly useful to examine the corolla lobes of the two South American species that had thus far been retained in Vernonia on the basis of the pollen and habit of the plants. Their position in Vernonia was always in question because of their remote geographical locations in Argentina, Brazil, Paraguay and Uruguay. The material was chosen from herbarium specimens that showed no indication of being preserved in alcohol or formalin. As shown in the figures (1G, H) no trace of multiple longitudinal ducts was seen. The character of the ducts is here regarded as a defining feature of the generic group of Vernonia, Vernonanthura and Trepadonia and the lack of the feature in the two South American species, combined with the remote geographical location is considered strong evidence that they do not belong in Vernonia. DNA of the two species has not yet been sequenced, and their proper disposition remains to be determined.
Alice Tangerini of the Dept. of Botany is thanked for assembling the plate of illustrations.
Vernonanthura brasiliana (L.) H.Rob., Venezuela: Lara: vicinity of Barquisimeto, 4 Jan 1923, José Saer 102 (
Vernonia angustifolia Michx., Arlington VA, corner of N. Calvert and 18th Sts, unvouchered garden plant: Aug. 2015, H. Robinson s.n.
Vernonia echioides Less., Brazil: Paraná, São João, in paludosis, 21 Mar. 1910, P. Dusén 9370 (
Vernonia incana Less., Paraguay: Dept. Neembucu, Dist. Yataity, Esterito, Jan 1975, M.A.Walter 118 (
Vernonia noveboracensis (L.) Michx., Washington, D.C., Roosevelt Island: 19 Aug. 2014, H. Robinson 14-1 (