Drosera anglica Huds. vs.
Drosera x anglica: What Is the Difference?
Keywords: evolution, taxonomy: Drosera anglica, Drosera x anglica,
Drosera linearis x Drosera rotundifolia.
Received: 1 March 1999
Drosera anglica Huds. is of hybrid, amphiploid origin (Drosera
linearis Goldie and D. rotundifolia L. being the parents) and
generally is not difficult to identify in its largely circumboreal distribution.
However, in the northern Great Lakes region of North America
where D. anglica occurs sporadically with D. linearis
and D. rotundifolia, problems occur when hybrids (D. x
anglica ) and the true species may be sympatric. Furthermore, there
is evidence that amphiploidy and therefore speciation is ongoing in the
area. A discussion of the origin of D. anglica, some ecologic factors,
problems with identification, and finally suggestions for distinguishing
between the species and hybrid is presented.
Introduction And Background
Drosera anglica Huds. is a north temperate to circumboreal species
that can be found in appropriate habitat in much of the northern tier
and some sub-tier of the United States and in southern Canada, extending
into Eurasia as far south as southern Europe (Schlauer, in litt.),
and into Japan and the Hawaiian island of Kauai (Diels, 1906; Wynne, 1944).
There has been some nomenclatural confusion in the past with D. longifolia
being used at times either synonymously or in precedence. The latter
has been recently resurrected in one flora and Cheek (1998) has applied
for rejection of D. longifolia altogether (see the cited paper
for a detailed history and reasons for suggested rejection). I will use
what I also regard as the preferred combination, D. anglica.
The presence of the species on the Alakai Swamp on Kauai in the Hawaiian
archipelago is of interest (Mazrimas, 1987; Gon, 1994; G. Newman, in
litt.) since the species is considered northern and Hawai'i is generally
tropical. In fact the bogs in which the plants occur are at 1200 to 1800
m (4000 to 6000 feet) elevation. Therefore, the local climate is not truly
tropical in these specific locations. There is no actual frost as there
is in wintertime abundance in the more common D. anglica habitat,
but nocturnal winter temperatures often descend to just above freezing,
and summer nights are cooler as well. Approaching winter dormancy in northern
continental habitats, the plant forms a tight winter bud (hibernaculum)
at ground level. This does not occur on Kauai, but during partial dormancy
new leaves are far shorter and atypical in appearance. The plants are
also generally smaller in this location and the smaller size and winter
behavior are a constant in plants grown from seed in temperate North America.
This article will concentrate on D. anglica as it is found in
the northern Great Lakes region of North America, particularly in northern
Michigan (including the upper peninsula). This area is of interest because
there is strong evidence that the species is of hybrid origin and this
is the only area where the putative parents, D. linearis Goldie
and D. rotundifolia L., can be found easily with D. anglica
(not uncommonly in the same fen) (Wood, 1955; personal observation). Interestingly,
other hybrids may be found including D. x obovata (D.
anglica x D. rotundifolia) rarely, and even the hybrid between
D. anglica and D. linearis recently has been identified
in nature (Schnell, 1995a). Finally, the hybrid D. x anglica
is also found and this can create immense confusion. So seemingly
difficult are D. anglica and D. x anglica to tell
apart that Voss, in his recently completed Michigan Flora (Part II 1985),
has decided that all the Drosera anglica-like plants in Michigan
should be referred to as D. x anglica for simplicity's sake.
Of course, this conclusion should not be applied in areas where D.
anglica and D. linearis are not sympatric since D. x
anglica would be impossible in that situation. I hope to show that
the two can be discerned, even in the field, where D. anglica and
D. x anglica are sympatric.
The species and hybrid in the area we will be discussing occur in a habitat
best described as a marl fen (Figure 1; see also description and photos
in detail in Schnell, 1980, 1982). Scattered across most marl fens in
this region one finds variably sized hummocks of sandy peat and Sphagnum
mosses (Figure 2). These little islands in the very wet sandy, marly peat
of the fen may vary from centimeters to several meters across, the latter
supporting shrubs and small trees. The fens are generally surrounded by
a "shoreline" of similar constitution as the hummocks, and then
dense forest. These hummocks and borders are usually acid in reaction
while the marl flats are basic to circumneutral. Of the Drosera
we are considering, D. linearis grows preferably in shallow water
(1-2 cm) over the marl flat, although occasional plants can be found growing
on the hummocks and even on wet, decaying logs. Drosera rotundifolia
grows most often on the tops of hummocks and above waterline on the
sphagnous fen margins. Drosera anglica and D. x anglica
usually can be found at the bases of hummocks or fen margins, at or
near the waterline--an intermediate position. After careful searching,
I have found that most of the upper Michigan fens have at least a few
to relatively many D. x anglica and fewer contain D.
anglica, but those that do often have them in abundance. The latter
are found more easily in the many fens of the eastern half of the upper
We must consider some of the breeding activity of these Drosera.
In habitat, the plants begin flowering more or less synchronously in late
June to early July. There is a raceme of flowers on the flower stalk (peduncle)
and these open and close daily in succession. Each flower opening that
day does so by mid-morning (if a bright, sunny day) and then closes by
mid-afternoon, and that is it for that particular flower. If a still unknown
pollinating agent has not acted in that brief time, the flower undergoes
self-pollination as the petals close and press the pollen-bearing anthers
against the stigma. Thus, seed is assured, even if not cross-pollinated.
The seedpods rapidly expand to 3-4 mm and yield mature seed by late August
into early September.
Also to be considered is the fact that hybrids or all North American
Drosera are sterile (Wynne, 1944; Wood, 1955; Cheek, 1993; Schnell,
1995b). The hybrids cannot breed with each other or the parents, in contrast
to the well known opposite situation with Sarracenia, for example.
In fact this sterility rule can be used as evidence in certain taxonomic
problems, such as determining whether D. filiformis var. filiformis
and D. filiformis var. tracyi should be considered as
varieties of one species or two separate species. Since the hybrid between
the two is quite fertile, this points to an infraspecific placement, as
it is usually classified (Schnell, 1995b).
In 1955, Wood detailed a compelling argument for the hybrid origin of
D. anglica. Noting that the species had a chromosome count of 2n=40
whereas all other northern Drosera were 2n=20, he hypothesized
that chromosome doubling had occurred in a hybrid in order to overcome
the sterility barrier in northern Drosera hybrids. (Those unfamiliar
with the n, 2n and x chromosome number designations as well as meiosis
vs. mitosis may wish to consult the appropriate chapters of a basic biology
or botany text.) Working out of the Douglass Lake University of Michigan
Biological Station, he studied the Drosera of several bogs and
fens in Michigan over several years. He noted the presence of D. anglica
as well as D. x anglica in several locations and deduced
that the parent plants were likely D. linearis and D. rotundifolia
based on sympatry and morphology.
How does a sterile hybrid of D. linearis and D. rotundifolia
become a fertile species? Studies indicate that the sterile hybrid
chromosome count is 2n=20 as expected, and the same as all other northern
Drosera species. If one examines the early flower buds of such
sterile hybrids by dissecting out anthers and ovaries and doing squash
preparations or microscopic sections early in development, one notes that
the special kind of cell division at certain stages of the development
of pollen and ovules known as meiosis is highly disturbed. Both meiosis
and mitosis are a precise sort of genetic dance in which chromosomes pair,
divide and then disjoin in a highly even manner to produce new nuclei.
But in sterile hybrids, some of the chromosomes lag, divide tardily, and
form bridges and fragments resulting in highly abnormal nuclei, all of
which can be observed by staining and microscopically examining tissue.
It is no wonder that such hybrids are sterile.
However, if some little-understood accident results in retardation of
meiosis altogether, fertile pollen and ovules with an unreduced chromosome
number occur which can then result in viable embryos and seed. The chromosome
number of the seed embryo and resulting seedling and plant has now doubled.
This process is known as allopolyploidy, or more often by the synonym
amphiploidy (e.g. Grant, 1981; Briggs & Walters, 1997). Since
the amphiploid plants chromosomes may now pair up properly with
the equivalent from the same contributing parent plant during meiosis,
the now amphiploid plant is capable of producing viable seed normally
generation after generation. Generally, such amphiploids are then recognized
as species rather than hybrids.
This then creates a problem for us in the field where parents, hybrids
and amphiploid species occur together: How do we easily tell the amphiploid
species from the hybrid in a consistent way? Wood (1955) accomplished
this by using chromosome counts of root tip (chosen because of active
growth and many mitoses being present) squashed and stained preparations
and squashes of developing anthers and ovules. He also determined that
the flat surface epithelium cells and stoma guard cells of leaf epithelium
differed in size, the species generally being larger in this respect,
presumably due to a greater chromosome complement than the hybrid. Because
the species had larger cells and nuclei, Wood was able to measure these
and separate the plants for his research. Having found and discerned the
D. x anglica hybrid in several northern Michigan fens, he
described it, naming it by formula (D. linearis x D. rotundifolia).
But he was not always certain of separating the hybrid and species from
each other by inspection of whole plants in the field.
Next, Wood discovered something of great evolutionary interest--The origin
of D. anglica is apparently polytopic; that is, it has occurred
by amphiploidy of the hybrid in more than one place, and this process
is probably ongoing. This phenomenon has been noted in several other non-carnivorous
genera and species (e.g. Wood, 1955; Grant, 1981; Briggs and Walters,
1997). Likely, the unknown stimulus for amphiploidy has worked and is
working in several different fen locations. Wood was able to deduce this
by noting a very few clusters or single plants of the fertile species
in large fens with many thousands of other Drosera species and
a even a few of their hybrids, and also noting that small populations
or individual plants of D. anglica (species) were often in fens
located many miles apart.
One wonders how D. anglica has developed such a wide present day
distribution nearly around the world while one of its evolutionary parents,
D. linearis, has remained so localized to the Great Lakes region
(but with a few disjunct populations in the Canadian maritimes and at
least one small population in northern Maine (Diels, 1906; Wynne, 1944).).
Did D. linearis at one time have a greater distribution than at
present and is receding into its present redoubt and perhaps further in
the future? Marly fens to which D. linearis seems confined are
very fragile habitats. I have seen several Lake Huron shoreline beach
pool fens destroyed in one season by severe winter storms and dune blowouts.
Or have plants of D. anglica simply been distributed widely away
from the present Great Lakes area to pioneer in suitable habitats nearly
around the world? Drosera anglica is ultimately a much more flexible
species in its habitat requirements than D. linearis in my observations
in the field and in cultivating the material. If distribution is the factor,
what were or are the carrying agents? Seed on bird feet, as the postulation
of plovers bringing propagules of D. anglica from Alaska to Kauai.
Or was it the prevailing winds bringing seed from Japan to the Alakai
Swamp (Mazrimas, 1987; Gon, 1994)? We do not know.
Differentiating Drosera anglica From Drosera
I conclude this paper by listing and briefly discussing ways to tell
D. anglica from D. x anglica. I will discuss the
most technical and complex methods that have been used first, then work
down to some more easily accomplished field methods. The most technical
procedures are of course most definitive at this time. However, they often
require sophisticated equipment and the processes themselves are usually
beyond the expertise of even the most dedicated amateur and often many
professional botanists. Some molecular biological procedures, such as
DNA, isoenzyme and FISH (fluorescent in situ hybridization), have not
yet been recorded for this problem, but have good potential, especially
Highly Technical Procedures
1. Microscopic sections prepared by standard histotechnological methods
of developing flower buds with staining to discern cells and features,
and looking particularly for developing anthers and ovaries to evaluate
meiosis. One searches for abnormal chromosome segregation and homologous
pairing with lagging and unmatched chromosomes and fragments (e.g.
Grant, 1981). Technical help is required for making the slides, and considerable
experience in evaluating them under the compound microscope.
2. Microscopic examination of stripped or peeled epithelium of the leaf
undersurfaces (to avoid glands) (Wood, 1955). Because of amphiploidy,
epithelial pavement and guard cells of stomata in D. anglica are
larger than those of the hybrid. These cells can be measured by planimetry.
3. Somewhat more accessible than above but requiring manual skill, experience,
stains, a compound microscope and often some luck, one can prepare squashes
of root tips (where there is a lot of mitosis going on) and even developing
anthers and ovaries of dissected flower buds to count and evaluate chromosomes.
In this case, we are interested in the 2n=20 vs. 2n=40 status of the root
tip cells, and evaluating meiosis in good preparations of the flower parts
cells. There is a huge body of scattered literature on how to do this
so I will not list it here. We are overdue a current single volume describing
the most prominent methods in detail with critiques.
Less Technical Procedures
4. The "less technical" here is relative since at least one
stain and a compound microscope (Do we not all carry one in our vans while
in the field?) are required. However, the procedure is not difficult to
do and evaluate. Pollen staining is based on the premise that viable or
living pollen grains capable of fertilizing an ovum will take up certain
stains while many sterile hybrids of plants produce only empty or incompletely
staining grains. A good reference for pollen viability procedures is Kearns
and Inouye (1993) who evaluate several of the pollen staining procedures
and find some lacking and/or little controlled on exactly how useful they
are. It is probably relative and some are likely more useful in certain
plant groups than others. I have had considerable experience with the
stain lactol phenol cotton blue on Sarracenia and Drosera,
using known and unknown hybrids compared to species, and find that it
is useful and consistent. One simply dusts a small amount of pollen (do
not overdo it or you will exhaust your stain and get false negative grains)
on the center of a clean, dry glass microslide, add a drop of the stain,
mix the stain and pollen thoroughly with a one-time use toothpick and
put a coverslip over the preparation. Viable grains of species and fertile
hybrids (e.g. as in Sarracenia) stain a deep blue across the entire
grain. Grains of sterile hybrids are empty and take no or far less stain.
A few hints not mentioned in the usual procedure instructions: After covering
your preparation with the coverslip, let it set for about four to six
hours for the stain to absorb. Secondly, to get used to what you are looking
for, try a few known fertile related species for comparison and also control.
Finally, remember nothing is 100% in testing. There will be some (usually
less than 5%) empty grains in perfectly fertile species, and a very few
grains staining positive in sterile hybrid preparations.
Procedures More Adaptable For Field Use
5. The next consideration is leaf shape. In his 1955 paper, Wood states
that D. anglica and D. x anglica cannot be distinguished
in the field by leaf shape, but a page later goes on to say that the leaves
of D. x anglica have more the shape of another hybrid, D.
x obovata (D. anglica x D. rotundifolia)--that
is, obovate-spatulate--while the species D. anglica is linear-spatulate
(Figures 3 and 4). Actually, both statements are true! On average when
examining many leaves of many plants, the hybrid D. x anglica
is indeed distinguishable from D. anglica (as is the case when
pulling three representative leaves of each taxon for Figure 4 out of
hundreds collected and pressed). But as is often the case in statistical
situations, there is overlap in single cases and individuals may present
a problem. To further confound the situation, D. x obovata is
occasionally found with our two problem taxa in the same fen. Usually,
when you find only a few widely scattered or small clumps of anglica-like
plants in a fen, you are more likely dealing with D. x anglica
since when D. anglica is present, it is usually in relatively
large clumps and/or numbers. Leaf shape can be helpful but its limitations
must be appreciated.
6. One can take advantage of the fertility of the species and the sterility
of the hybrid to observe whether Drosera anglica-like plants have
maturing seedpods, and even collect seed to compare with the fine Wynne
(1944) drawings. If you are bogging in the spring or early summer before
seedset, you can flag your plants and examine them later in the season.
7. I have noted that the species has a consistently larger corolla than
the hybrid, similar to the epithelial cell sizes mentioned in item 2 above.
Drosera x anglica measures 6-7 mm across, while D. anglica
is 8-10 mm. This is very useful If you catch the plants in flower.
8. Comparable to item 7 above, the peduncles or flower stalks are of
different thickness (see Figure 5,6,7): The hybrid consistently measures
1.0-1.2 mm in thickness while the species is 2.0-2.2 mm. This is also
In summary, what do I do? Technically, I do indeed use pollen viability
studies on occasion. Leaf shape is also very helpful when correlated with
other factors and taken in perspective. Of course, seed production is
quite definitive in the field and horticulturally. But I have found that
there are flowers and/or peduncles nearly all summer, and sizes of these
are the most helpful.
Briggs, D. and Walter, S.M. 1997, Plant Variation and Evolution, Cambridge
University Press, 512 p.
Cheek, M. 1993, Notes on Hybrids in Drosera, Kew Magazine 10:138-144.
Cheek, M. 1998, Proposal to Reject the Name Drosera longifolia
(Droseraceae), Taxon 47:749-750.
Diels, L. 1906, Droseraceae, in Das Pflanzenreich, IV. 112, Heft 26,
Verlag von H. R. Engelmann (J. Cramer), Weinheim, Bergstrasse.
Gon, S. 1994, The Hawaii Population of Drosera anglica--Tropical
Twist on a Temperate Theme, Carniv. Pl. Newslett., 23:68-69.
Grant, V. 1981, Plant Speciation, Columbia University Press, New York,
Kearns, C.A. and Inouye, D.W. 1993, Techniques for Pollination Biologists,
University Press of Colorado, Niwot. 583 p.
Mazrimas, J.A. 1987, Drosera anglica from the Alakai Swamp,
Kauai, Hawaii. Carniv. Pl. Newslett., 16:21-22.
Schnell, D.E. 1980, Drosera linearis, Carniv. Pl. Newslett.,
Schnell, D.E. 1982, Notes on Drosera linearis Goldie in Northeastern
Lower Michigan, Castanea 47:313-328.
Schnell, D.E. 1995a, A Natural Hybrid of Drosera anglica Huds.
and Drosera linearis Goldie in Michigan, Rhodora 97:164-170.
Schnell, D.E. 1995b, Drosera filiformis Raf.: One Species or
Two? Carniv. Pl. Newslett., 24:11-15.
Voss, E.G. 1985, Michigan Flora, part II, Cranbrook Institute of Science
(Bulletin 59) and University of Michigan Herbarium, Ann Arbor, 724 p.
Wood, C.E. 1955, Evidence for the Hybrid Origin of Drosera anglica,
Wynne, F.E. 1944, Drosera in Eastern North America, Bull. Torrey
Bot. Club, 71:166-174.