The present study was undertaken to provide modern distribution maps and a workable key for the Iowa milkweeds. The genus Asclepias has not been separately studied in Iowa, so far as\u27\u27 the authors can ascertain. In the most recent Iowa flora, Conard (1951) lists 16 species. In the present work 15 species are recognized. Appreciation is extended to Dr. Robert Thorne and Dr. Richard Pohl for the use of specimens from the herbaria of the State University of Iowa and Iowa State College. In addition, material from the herbarium of Grinnell College has been studied. In all, approximately 1200 sheets of Asclepias were examined

Nicolson, Dan

Russell, Norman H.

English

Name not available

The Genus Asclepias in Iowa

University of Northern Iowa

Proceedings of the Iowa Academy of ScienceVolume 62 | Annual Issue Article 221955The Genus Asclepias in IowaDan NicolsonGrinnell CollegeNorman H. RussellGrinnell CollegeCopyright © Copyright 1955 by the Iowa Academy of Science, Inc.Follow this and additional works at: https://scholarworks.uni.edu/piasThis Research is brought to you for free and open access by UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academyof Science by an authorized editor of UNI ScholarWorks. For more information, please contact scholarworks@uni.edu.Recommended CitationNicolson, Dan and Russell, Norman H. (1955) "The Genus Asclepias in Iowa," Proceedings of the Iowa Academy of Science: Vol. 62: No.1 , Article 22.Available at: https://scholarworks.uni.edu/pias/vol62/iss1/22The Genus Asclepias in Iowa By DAN NICOLSON AND NORMAN H. RUSSELL INTRODUCTION The present study was undertaken to provide modern distribution maps and a workable key for the Iowa milkweeds. The genus Asclepias has not been separately studied in Iowa, so far as'' the authors can ascertain. In the most recent Iowa flora, Conard ( 1951 ) lists 16 species. In the present work 15 species are recognized. Ap-preciation is extended to Dr. Robert Thorne and Dr. Richard Pohl for the use of specimens from the herbaria of the State Univer-sity of Iowa and Iowa State College. In addition, material from the herbarium of Grinnell College has been studied. In all, approxi-mately 1200 sheets of Asclepias were examined. Woodson ( 1954) has been followed in the nomenclature accepted for the milkweeds of Iowa. The key to species was prepared from herbarium material, but the keys to Asclepias in Woodson ( 1954) and Gleason ( 1952) proved of especial aid in the determination of which characters had the greatest taxonomic value. The distribution maps represent, in every case, specimens actually examined. This practice of not including literature reports has not significantly diminished the apparent ranges of any of the species, with the exception of A. meadii, which is discussed below. An attempt has been made to render the following key especially useful in the identification of dried specimens by omitting such characteristics as corolla color and the presence or absence of a marginal lobe on the corolla hood. Both of these characteristics are useful in identifying living plants but are very difficult to dis-tinguish on herbarium material. KEY TO Asclepias IN IOWA 1. Hoods without horns. 2. Corona distinctly on. a short column; peduncle thin, 2-4 cm. long ........................ A. hirtella (Pennell) Woods. (Fig. 1) 2. Corona sessile; peduncle thick, nearly obsolete to 15 cm. long. 3. Umbels lateral and usually numerous; plants mostly taller than 3 dm ......................... A. viridiflora Raf. (Fig. 2) 3. Umbels terminal and solitary; plants usually less than 3 dm. in height ........................ A. nuttalliana Torr. (Fig. 3) I. Hoods with distinct horns. 4. Hoods equaling or slightly surpassing gynostegium; horns definitely surpassing the hoods. 5. Leaves below inflorescence alternate; plant hirsute ....... . ................ A. tuberosa L. subsp. interior Woodson (Fig. 4) 5. Leaves below inflorescence opposite or whorled; plant never hirsute. 6. Corona and hoods truncate; hood tubular. 211 1Nicolson and Russell: The Genus Asclepias in IowaPublished by UNI ScholarWorks, 1955212 IOWA ACADEMY OF SCIENCE 7. Leaves petiolate, cuneate at base A. exaltata L. (Fig. 5) [Vol. 62 7. Leaves sessile, broadly rounded at base ............... . ........ A. amplexicaulis Sm. (Fig. 6) 6. Corona and hoods not truncate; hood scoop-shaped. 8. Leaves linear and whorled ........................ A. verticillata L. (Fig. 7) 8. Leaves lanceolate and opposite ....................... . A. incarnata L. subsp. incarnata (Fig. 8) 4. Hoods definitely surpassing gynostegium; horns not surpas-sing the hoods. 9. Lower leaf surface pubescent. 10. Hoods 11-15 mm. long; summit of hood elongate; pedicels very pubescent ................ A. speciosa Torr. (Fig. 9) 10. Hoods 5-8 mm. long; summit of hood ovate or oblong; pedicels slightly pub-escent. 11. Plants large ( 4"20 dm. tall) ; prin-cipal leaves 6-18 cm. long; venation pinnate. 12. Umbels several and always axil-lary; venation always strongly pinnate; summit of hood obtuse ........................ A. syriaca L. (Fig. 10) 12. Umbels one or two and terminal or in upper two leaf axils; vena-tion less strongly pinnate, vary-ing toward .net-like; summit of hood acute ........................ A. purpurascens L. (Fig. 11) 11. Plants small ( 1-3 dm. tall) ; princi-pal leaves 3-7 cm. long; venation net-like ........................ A. ovalifolia Dene. (Fig. 12) 9. Lower leaf surface glabrous. 13. Leaves petiolate and with cuneate bases; plants usually with one whorl of four leaves ........................ A. quadrifolia Jacq. (Fig. 13) 13. Leaves subsessile and with subcordate bases; plants with strictly opposite leaves. 14. Plant 2.4 dm. tall; leaves with scabrous margins; leaves 5-7 cm. long ....... . ................ A. meadii Torr. (Fig. 14) 14. Plant 6-9 dm. tall; leaves with glabrous margins; leaves 9-15 cm. long ....... . ................ A. sullivantii Engelm. (Fig. 15) 2Proceedings of the Iowa Academy of Science, Vol. 62 [1955], No. 1, Art. 22https://scholarworks.uni.edu/pias/vol62/iss1/221955] ASCLEPIAS IN IOWA 213 Figuires 1-8 Fig. 1. Iowa range of Asclepias hirtella (Pennell) Woods. Fig. 2. A. viridi-flf!Tfl ~.af. Fig. 3. A. nuttalliana Torr. Fig. 4. A. tuberosa L. subsp. interior Woodson. "Fig. 5. A. exaltata L. Fig. 6. amplexicaulis Sm. Fig. 7. A. verticillata L. Fig. 8. A. incar-nata L. 11ubsp. inearnata. DrscussroN Though the milkweeds are, in general, quite distinct and easy to distinguish, a number of problems were met during the course of the present study. Some of the more significant of these are dis-cussed below. In one respect, Woodson ( 1954) was not followed. It was found very difficult to distinguish Asclepias tuberosa L. subsp. terminalis Woodson. The authQcs feel that, at least so far as Iowa material is concerned, there is lktle evidence for according this form subspecific 3Nicolson and Russell: The Genus Asclepias in IowaPublished by UNI ScholarWorks, 1955214 IOWA ACADEMY OF SCIENCE [Vol. 62 Figures 9-15. Fig. 9. Iowa range of Asclepias speciosa Torr. Fig. 10. A. syriaca L. Fig. 11. A. purpurascens L. Fig. 12. A. ovali/olia Dene. Fig. 13. A. quadri/olia Jacq. Fig. 14. A. meadii Torr. Fig. 15. A. sullivantii Engelm. rank and have therefore not recognized it. All Iowa specimens of A. tuberosa have been placed in Woodson's subspecies interior. A number of varieties, based upon particular leaf shapes have been recognized in Asclepias viridiflora. Woodson believes that this variation is not sufficiently consistent to justify recognition of these varieties, and the present authors have agreed with this judgment. Color forms have not been included in the key, though corolla color often varies intra-specifically, particularly in A. tuberosa. A characteristic of value in identification of milkweeds is the presence or absence of marginal lobes on the corolla hoods. A. syri-4Proceedings of the Iowa Academy of Science, Vol. 62 [1955], No. 1, Art. 22https://scholarworks.uni.edu/pias/vol62/iss1/221955] ASCLEPIAS IN IOWA 215 aca, A. ovalifolia, A. quadrifolia, and A. meadii have these lobes; A. speciosa, A. purpurascens, and A. sullivantii do not. Hybridization between milkweeds seems to be very rare in Iowa. Only a single instance was found of putative hybridization. In the ISC herbarium there is a set of milkweed collections by Ada Hayden from a wet prairie meadow in Clay County, which includes material of both A. syriaca and A. speciosa. A number of these specimens have corolla hoods intermediate in length between those of A. syriaca (5-8 mm. long) and A. speciosa (11-15 mm. long) and appear intermediate in other characters, such as leaf pubescence. Woodson ( 1954) mentions that spontaneous hybrids have been found between these two species and that the cross was performed experiimentally by 0. A. Stevens at the North Dakota Experiment Station. The two rarest Iowa Asclepiads are Asclepias nuttalliana and A. meadii. We have four reports for A. nuttalliana, a tiny milkweed often found with A. ovalifolia. Woodson mapped two Iowa locations for A. meadii and two more collections in Illinois just across the Mississippi River. Only one specimen from Iowa was seen by the present authors. It is deposited in the Iowa State College herbarium and has been seen and verified by Woodson, as have most of their other specimens of Asclepias. Literature Cited Conard, H. S. 1951. Plants of Iowa, 7th Edition. Gleason, H. A. 1952. The New Britton and Brown Illustrated Flora of the Northeastern United States and Adjacent Canada. Three Vols. The New York Botanical Garden. Woodson, R. E. Jr. 1954. The North American species of Asclepias L. Ann. Mo. Bot. Gard. XLl:l-211. GRINNELL COLLEGE GRINNELL, IowA 5Nicolson and Russell: The Genus Asclepias in IowaPublished by UNI ScholarWorks, 1955

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The Genus Asclepias in Iowa

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