A Geographical Analysis of Morphological Characters for Taxus contorta (Taxaceae)
R. W. Spjut
World Botanical Associates
Taxus contorta Griffith has been clearly established as a distinct species for the yew in Northwest Himalayas based on morphological data (Spjut 2007a, 2007b) and molecular data
Data in this table are summarized for eight characters
scored from 45 herbarium specimens studied of Taxus contorta; the herbarium collection data, the SR and MC data
were extracted from another ms on the phytogeography of Taxus prepared in 2001, which
was submitted to Sida for peer review. Characters include the number of stomata
rows (SR), the number of epapillose cells along the
abaxial leaf margin (MC), and other less taxonomically useful characters—abbreviated
as PAP, CSL,
SR: Stomata Rows, number of stomata rows in a stomata band. Stomata count can vary on a plant; for example, Stewart 8414 was represented by three specimens from three different herbaria in which stomata were found to be either in 5, 6 or in 7 rows per band. A difference of 2 rows on a plant is not uncommon, and even a difference of 1 row can be found in stomata bands on a leaf, the result of which is expressed as an intermediate value; for example, a leaf with 7 to 8 rows of stomata is indicated as 7.5.
Number of stomata rows increase from west to east in the
The higher counts in plants of T. contorta from
MC: Marginal Cells, number of marginal epidermal cells on abaxial surface without papillae. The leaf margin in T. contorta—where the adaxial and abaxial surfaces come together—is difficult to identify due to the nature of the broadly round margins that characterizes the species. The leaf margin is determined by where the epidermal cells are smallest in width and in height, and is usually marked by a single mammilla. From an evolutionary view, the marginal border of cells lacking in papillae appear to be a modification of the adaxial epidermal cells as the species adapts to changes in climate.
The abaxial marginal cells of T. contorta, along with the number of stomata rows, are
remarkably consistent in having 4–5 rows of epapillose
cells for many specimens from the NW Himalayas. This is in contrast to specimens of T. contorta from the Central Himalayas
that generally have a wider marginal border of cells, as seen in specimens
PAP: Papillae, position of papillae on epidermal cells, classified as either medial or marginal. Medial papillae are oriented towards the center of the cell, and if multiple papillae develop, papillae often lie opposite each other in two or more rows. Marginal papillae, on the other hand, are nearer the cell wall, and in specimens with multiple rows of marginal papillae on a cell, the papillae appear alternate instead of opposite.
The difference in orientation of papillae is believed to be a genetically determined trait, which can vary on a leaf. Some specimens are noted to have marginal papillae in one region of the leaf and medial papillae in another area of the leaf. The ancestral condition is believed to be marginal in which there is little differentiation between the midrib and stomata band as exemplified by Taxus wallichiana; however, Taxus wallichiana var. yunnanensis is distinguished from the typical variety by its medial orientation of papillae on epidermal cells. The differences in medial vs. marginal papillae in T. contorta are believed to be related to ecological factors. Collection data suggest a partial correlation with altitude.
CSL: Cell Shape in
Longitudinal section, as actually seen on the abaxial leaf surface in the
marginal region. The abaxial marginal
cells may appear short and somewhat pentagonal, or somewhat hexagonal,
referred to as diamond shaped in the table, or they may appear longer and
narrowed towards one end, referred to as rectangular (rect)
in outline, but if notably rounded at both ends and also relatively narrow,
the cells are then described as fusiform (this character attribute was not
present on marginal cells for specimens of T. contorta, although the midrib cells are fusiform). Taxus canadensis and T. sumatrana are examples of species
with leaves that have relatively long rectangular epidermal cells along the
abaxial margins. Specimens of Taxus contorta from
CSX: Cell Shape in Cross section. The epidermal cells may appear globose (very rare, British Isles, T. baccata), elliptical (common in T. baccata and allies), rectangular to isodiametrically square (common in T. wallichiana and allies), and isodiametric rounded-angular (occasional in T. contorta). Shape of epidermal cells in x-section is difficult to evaluate as it depends on making good x-sections of leaves, and because the differences are often relatively minor. Taxus contorta is similar to T. baccata in having short elliptical cells, in contrast to smaller and wider elliptical cells of T. cuspidata and its allies. However, when color of epidermal cells is taken into consideration, as seen on older herbarium specimens, the differences between T. contorta and T. wallichiana are more easily discernible.
Arrangement (phyllotaxy). Leaves in Taxus
are spirally arranged. Differences in Taxus
phyllotaxy are related to leaf density along the branch, a feature that
is more easily observed than defined by mathematical ratios. Each species generally has a distinctive
arrangement of leaves. In other words,
phyllotaxy is constant for a species.
An example is shown for two specimens of Taxus biternata placed one on top of another so that the branch
of one from
However, phyllotaxy may be distorted by leaf spread (LS), which is affected by direct exposure to light (leaf orientation) as explained below, or it can vary in more complex species such as T. baccata for which many varieties have been recognized.
In the Euro-Mediterranean, T. recurvata is distinguished from T. baccata by radial leaves appearing notably arcuate (recurved) in which the leaves curve downwards, and by their difference in color between the abaxial and adaxial surfaces (discolorous). Leaves in T. baccata are more distichous, concolorous, glaucous, and upturned (towards the light source). Thus, for these Euro-Mediterranean species, leaf arrangement, leaf spread, and leaf color are correlated.
In Taxus contorta, leaf arrangement varies from somewhat radial to somewhat distichous. The differences between radial and the distichous types of T. contorta are difficult to evaluate taxonomically. Leaves on young branchlets appear more distichous than on older branchlets, while older branches may also lose some of their leaves. Nevertheless, data in the table suggests that leaf orientation in Taxus contorta is variable throughout its range.
LS: Leaf Spread
orientation), the angle at which leaves spread outwards from branches. This is influenced by amount of exposure to
light and to what extent plants are adapted to growing in shade vs. sunlight. Taxus
contorta is an understory tree in spruce, pine and oak forests of the
western and central
From the data in the table, it can be seen that leaves
from specimens collected in
PAR: Parenchyma Cells, the shape of cells in spongy parenchyma layer of leaves, varies from elliptical, globose, to angular in T. contorta. This is in contrast to bone-like cells of T. wallichiana and related species. Old herbarium specimens of T. contorta appear to have heavy deposits of resinous-like (terpenoid) substances on parenchyma cell walls, but this is not evident in young specimens such as those that were approximately one-year old in which the cells all appear clear. Nevertheless, the spongy parenchyma cells still appeared loose without any adhesiveness as seen in older dried specimens. Thus, in relatively recently collected specimens of T. contorta, the loose arrangement of the globular cells is still evident, a features that characterizes this species. Taxus baccata is recognized to have similar parenchyma cells, and the differences between these species would seem to be leaf curvature (straight vs. falcate leaf blades).
Species vs. varietal designation.
Species status in Taxus depends
on specimens either having a combination of unique features, or a large
number of shared features among many specimens over a wide geographic
area. In the NW Himalayas, the
majority of specimens have relatively long, narrow, straight leaves with
rounded margins, 5-8 (-10) stomata rows per band and an abaxial margin of
(0-) 3–6 marginal cells. All the
specimens had hardened resinous parenchyma cells in the leaf mesophyll and a
papillose midrib on the abaxial surface.
The presence of hardened parenchyma cells in the leaf mesophyll is
unique to Taxus contorta. Most of the other features mentioned above
also help separate T. contorta from
An example of a taxon with a much narrower
range in occurrence was first recognized as a species based on a specimen by
One exception, however, soon appeared in the
November 1997 issue of Novon
where T. fuana was described as a
new species (http://www.fna.org/china/novon/lifu7-3.htm); in a commentary the
authors (Nan Li and R. R. Mill in Li & Fu 1997) indicated there was an
illustration for the species in the Flora of China (Cheng & Fu 1978). Whereas
all the specimens cited by the authors clearly belonged to T. contorta, the illustration
they mentioned in Cheng & Fu
(1978) was clearly Spjut's T. mucronata. In
Taxus contorta Griffith has been clearly established as a distinct species for the yew in Northwest Himalayas based on morphological data (Spjut 2007a, 2007b) and molecular data (Amin Shah et al. 2008). In addition to the morphological separation between the Central and East Himalayan T. wallichiana from the Northwest Himalayan T contorta, molecular data that includes specimens from the Baccata Alliance reported three distinct lineages; thus, T. contorta is clearly separated from yews of the Euro-Mediterranean Region. Unfortunately, Amin et al. (2008) chose to uphold an illegitimate name (T. fuana), while the nomenclature at the NPGS/GRIN has been based on an unsubstantiated and outdated taxonomic concept of a single Himalayan species of yew (T. wallichiana).
Upon further study of the leaf fragments from additional specimens at BM, and at K, the anatomical distinction between T. contorta and T. mucronata could not be upheld due to intermediates; therefore, the new taxon (T. mucronata Spjut ined.) was recognized only as a variety, T. contorta var. mucronata (Spjut 2007b).
Additional Reference noted here: Shah, A., D.-Z. Li, M. Möller, L.-M. Gao, M. L. Hollingsworth and M. Gibby 2008. Delimitation of Taxus fuana Nan Li & R.R. Mill (Taxaceae) based on morphological and molecular data Taxon (http://www.ingentaconnect.com/content/iapt/tax/pre-prints/571shah).