Taxonomic keys and species descriptions are presented for 24 species and 55 varieties of Taxus (Taxaceae) in North America and Eurasia. 

This study is an outgrowth of my collecting activities for the National Cancer Institute (NCI) and affiliated drug discovery programs at universities (Perdue & Hartwell 1969; Hartwell 1976; Suffness & Douros 1979; Spjut 1985, Spjut et al. 1986, 1988, 1992, 2005).  The information on Taxus was extracted from three papers, two of which were peer reviewed in 2001, and includes figures and their numbers, which may not be in the same sequence as in the manuscripts.  These manuscripts were revised and submitted for peer review in May 2006, and finally published August 2007.  Information on Taxus was edited during the peer review period, and additional photos of herbarium specimens have been presented for all species and varieties.  Not all species proposed on the web pages have been published as additional study is recommended for the unpublished names.  Changes that have been made relate to reassignment of specimens as concepts of species and varieties were refined.  Occasional errors in specimen citations have been found and corrected. 

My research on Taxus began with a botanical review of T. brevifolia Nuttall under a cooperative agreement between the United States Department of Agriculture’s (USDA) Agricultural Research Service (ARS) and the NCI (Spjut, USDA Memorandum 1977).   This was needed to determine where the ARS could procure tons of stem-bark needed for clinical trials of the anticancer compound taxol.  Extracted bark from this plant was first discovered to have an antitumor activity in 1964, and interest in the elucidation and pharmacological activity of the active compound—taxol continued (Perdue & Hartwell 1969; Wani et al. 1971).  Taxol generally became a useful drug for treating ovarian and other cancers by 1990 (Kingston et al. 1990; Rowinsky et al. 1990; Cragg et al. 1993, 1996).  The need for taxol led to investigate alternatives for producing taxol (Perdue & Hartwell 1969; International Yew Resources Conference 1993; Croom 1995).   

Preliminary taxonomic reviews were made of species in the genus during 1986–1989 for the NCI and for universities who were applying for grants to isolate taxoids from Taxus spp.   My study increased in its scope in response to present papers at various conferences (e.g., Spjut 1992, 1993, 1998, 2000) for which peer-reviewed papers were solicited and finally published in Aug. 2007.  This study is based on numerous fresh specimens received from many areas in the world—including China. These were often sent via overnight mail through the USDA Forest Service, Purdue University, and Phyton Inc. (in Ithaca, NY, a tissue culture laboratory that  produces taxol, now Phyton Biotech which has since relocated to New Jersey).  

In November 1995, the ARS demanded that I discontinue research on Taxus.   Loans from various herbaria had to be returned prematurely, but specimens were annotated based on my understanding of the taxonomy at that time (June 1995–July 1996).  My research on Taxus, nevertheless, has continued—under World Botanical Associates.  Since March 1997, I have studied material at herbaria in the United States and Europe, and  obtained additional loans through the U.S. National Herbarium.  Additionally, type material has been kindly provided directly to me by curators from institutions in China, Scotland, France, and the United States.   Other leaf fragments have been received from scientists conducting molecular studies on yew in China, Pakistan, and in Switzerland.  More than 1,000 specimens have been studied throughout the natural range of the genus (temperate and subtropical America N of Panama, and Eurasia N of Wallace's Line).

The lack of clear reproductive isolation mechanisms in the genus Taxus supports the monotypic species concept (T. baccata L., Pilger 1903); however, this would have to
include the related monotypic Pseudotaxus (synonym Nothotaxus)¹, an endemic genus to China, distinguished by leaves having glaucous instead of papillose stomata bands, additional sterile scales in male strobili, and by the seed enclosed in a white instead of a red aril.   Nevertheless, Taxus is known from fossils dating to the Oligocene (Kvaček 1984), and there are related extinct genera—Paleotaxus and Marksea—that date back to the Jurassic (Florin 1958, 1963).  In view of this history and phytogeographical data on morphological features of extant Taxus (Spjut 1998, 2007a), the monotypic species concept for Taxus (Pilger 1903, 1913, 1926) cannot be upheld.

Taxus has been interpreted to include 7–12 species (e.g., Cope 1998; Farjon 1998, 2001; Rehder 1940, 1949; Silba 1986) based on morphological criteria that best fits geographical disjunct populations (Rehder 1940; Cope 1998); however, I recognize  24 species and 55 varieties—based largely on morphological differences.  Characters include the habit of the plant, branching pattern, leaf shape, leaf color, angle at which leaves spread, relative distance between leaves, shape of leaf epidermal cells in transverse (T) section, number of stomata rows, relative width of a marginal zone bordering stomata bands, the relative length of cone axis, and others.  As one gains familiarity with the genus, most species can be recognized by general morphological features. Leaf anatomical features are then used to verify the determination. 

References Cited Above: See Introduction to Taxus

  Key to Taxus Species Groups and Subgroups

                                        1.  Taxus wallichiana Group

                                                Subgroup wallichiana Group (North America, E India-SW China)

                                                Subgroup chinensis (India to Indonesia and The Philippines)

                                        2.  Taxus sumatrana Group (India to Indonesia and Philippines)

                                        3.  Taxus baccata Group (Euro-Mediterranean)

                                                 Taxus baccata Alliance (Euro-Mediterranean)

                                                 Taxus cuspidata Alliance (E Asia, E North America)

USDA ARS Memoranda/Letters by R. Spjut: Review and Proposed Research on Taxus.

1. Research Proposal on a Taxonomic Revision of Taxus, USDA ARS Memorandum from
Richard Spjut to Thomas S. Elias, May 25, 1995.  Includes Key to Species of Taxus by
R. Spjut dated April 24, 1995 and character list in DELTA format, 103 characters, 86 of which are morphological. 20 pages.

2. Letter from R. Spjut, USDA Beltsville Agricultural Research Center, to Kenneth Snader, Natural Products Branch, National Cancer Institute, NIH, Nov. 4, 1991.  2 pages.

3. Spjut, R. W.  1977 (July 14). Taxus brevifolia Nutt. (Taxaceae), nomenclature, description, distribution, ecology, comments, uses, other, references, and attachments—table 8 from Franklin, and Dyrness (1969), distribution maps.  USDA ARS Medicinal Plant Resources Laboratory, Beltsville, MD.  12 pp circulated to USDA lab scientists, NCI; content also incorporated into NCI solicitations for large collections of stem-bark, sent to prospective bidders in the Pacific Northwest, 1978–1990.