Polygala vulgaris, the Common Milkwort (Queendown Warren, Kent,
UK). These are
small flowers, the calyx wings of this species are 6 to 15 mm
The Polygalaceae are a widely distributed family of herbs, shrubs,
lianas (woody vines) and
trees. The genus we shall focus on here is Polygala, the Milkworts. The
photographs on this
page cover a number of chalky/heathy habitats in Kent, British
isles. In particular, the habitats
selected have two species: Polygala
the Chalk Milkwort, and Polygala
Common Milkwort. The heath Milkwort, Polygala
is also common in the British Isles,
but has not been recorded from the habitats examined here. Polygala amarella (Dwarf
Milkwort, formerly Polygala
is also found in the British isles, but is restricted to a very
Above: Polygala sp., Queendown Warren,
The flowers of Polygalacae are often described as 'pea-like'.
Certainly they have evolved
along similar lines since both have converged on the flag-keel
model: each flower has a flag to
advertise itself to pollinators and a keel which the weight of the
pollinator depresses to expose
the nectar and reproductive parts. However, such similarities are
superficial and the flower of
Polygalaceae are unique. They look rather like tiny flying
creatures, or winged squid. The
'wings' are calyx
formed by the inner lateral sepals which resemble petals (petaloid
sepals) and are therefore not equivalent to the wings in pea
flowers. These calyx
flags, advertising the flower to potential pollinators.
The single lower (median, abaxial) petal folds forwards on itself
to form a pair of pockets or
auricles which enclose the
anthers. This petal forms the keel. The lateral edges
this petal roll over to form a furrow which acts as a nectar cover. (In the Pea family,
Fabaceae, the lower keel is formed from a pair of petals). The
sides of this petal, just behind
the tips (which are folded to form the auricles) give out tubular
protuberances, forming the
crest. (The crest is therefore
formed of paired lateral and subapical structures). The crest
forms a landing
for the pollinator who pushes away the nectar cover which
depresses the keel (which has a true hinge in some species) to
allow access to the pollen and
nectar, the furrow formed by the keel now serving as a channel for
the pollinator's proboscis.
The auricle tips may also fold back to the tip of the keel and act
as levers, depressing the keel
(as has been described in Polygala
The two upper petals overlap one another and have strengthened
claws to form the structure
of the petal tube. (The 'claw' is the narrow basal part of a
petal). The tips of these petals
(blades) stand upright and may contain nectar guides and so act as
They also form an abutment, against which a pollinating bee pushes
with its head as it
proboscis is guided by the tube formed from the keel and dorsal
petal claws. The pollinator
pushing against this abutment, whilst standing on the crest, may
cause the lowering of the keel
and the opening of the flower. Whatever the exact mechanism
depressing the keel, only an
insect of suitable size and strength can operate the mechanism,
ensuring an insect of the right
size to effect pollination.
Common Milkwort, Polygala
Yocklett's Bank, Kent, UK. The flowers seen here
have short crests.
species of Polygala
picture above and the picture below show Polygala
the Common Milkwort
(specifically subsp. vulgaris). One way we can tell is
by the venation, or pattern of veins, on the
calyx wings. Notice how the veins anastomose (join together) in
such a way as to divide the
wing into a large number of encircled units called areolae.
have a pentamerous ancestor (a flower with whorls of five
floral formulae generally designate 5 petals, as in Ronse de
Craene (2010). It is stated that the
remaining four petals fuse to form the pair of dorsal petals.
However, microscopical studies by
(2010) showed that the two lateral (side) petals are suppressed
and either arrest
as primordia (small protuberances on the growing shoot) or do not
develop at all. (In this case
it is probably more correct to give 3 petals rather than 5 in the
floral formula). The nectary is a
ring of tissue around the gynophore (stalk of the ovary) which may
be most well developed
uppermost. The presence of the gynophore, which is a narrowing of
the ovary base, creates
space to hold the secreted nectar in a nectar
Polygala vulgaris, from Bilder ur Nordens Flora (Pictures of
the Nordic Flora) C.A.M. Lindman, 1917-1926. Image in the Public
Domain. 1. Flowering shoot (note that the lower leaves are smaller
and become more rounded or elliptical). 2. Whole flower; 3. Detail
auricles and crest; 4. Stamens and pollen spoon; 5. Androecium
(stamens); 6 sepals enclosing fruit capsule; 7. Fruit capsule
the small wing).
stamens of Polygalaceae have their filaments united to form a kind
of cup or
beneath the stigma which holds the stigma in place as the keel is
depressed. The filaments also unite, at some point, with the lower
petal of the
corolla. The anthers are unusual (in most Polygalaceae) in that
each opens via a
as illustrated below:
floral diagram for Polygala. The innermost petals
are in cyan. The 8 stamens (yellow)
are fused together and to the lower petal. Ancestrally, there
would have been 10 stamens (two
whorls of 5) but the lowermost (the abaxial antesepalous stamen,
i.e. opposite a sepal or, as in
this case, a bract) and the uppermost (adaxial antepetalous
stamen) are not formed (open
circles). The stippled bulges on the sides of the lower (abaxial)
petal represent the vestigial
lateral petals. Note the two dorsal petals complete the floral
tube. The ovary (pink central
structure) contains two compartments, each producing an ovule. The
petal-like lateral sepals
(calyx wings) are stippled blue, the remaining three sepals (one
median and adaxial, two
lateral) are more leaf-like (stippled green) and much smaller than
the petals. Bracts are in pale
green. (Adaxial refers to the side closest to the shoot axis
(green circle), abaxial that side
furthest from the axis).
The downward arrow indicates that the flower is zygomorphic with a
single axis of symmetry
(monosymmetric) and is a 'flag-flower', that is it delivers its
pollen to the underside of the
model of a pollen grain of Polygala vulgaris. The
pollen is zonocolporate. 'Zono' refers to the fact that the
apertures (pores) for the pollen tube to exit from are arranged
along the equator: the pollen tube will grow from one of these.
Colpi are the grooves or slits where the pollen wall is thinner
and in this case are arranged in sectors. 'Colporate' refers to
the fact that a pore occurs within each colpus. The poles are
apocolpial, meaning that they contain surface sculpturing
formed by the joining of the colpi margins. These apocolpial
regions contain windows or lumina where the wall is thinner.
There are between 8 and 22 colpi in Polygala, 11 in this
more than two obvious areolae is a characteristic feature of Polygala vulgaris. The two
venation patterns seen on the left above are characteristic of Polygala vulgaris subsp.
vulgaris. The third pattern, combined with the cuneate (cone-like
in contour) base is
characteristic of Polygala
subsp. oxyptera. The fourth pattern is
the Chalk Milkweed: There is a single pair of anastomoses between
central and two lateral veins towards the apex of the sepal. The
lateral veins give out
secondary veins which do not (or seldom) anastomose. The fifth is
an example from Polygala
the Heath Milkwort: the veins give out fewer secondary
branches. The last pattern on the right is an example from Polygala amarella: there are no
anastomoses at all and the three main veins (median and two
laterals) are very prominent and
the lateral veins give off only a few secondary veins on the
outside. These last two species can
also be distinguished from P.
by their much smaller flowers (and P.
has a well formed and compact rosette of basal leaves near where
emerges above ground). Heath Milkwort was described as by far the
most common milkwort in
Britain by Boswell Syme (1864). Although it prefers acidic soils,
its presence on calcareous
grassland can not be ruled out, since calcareous grassland
sometimes contains pockets of
more acidic soil.
stigma is divided into two lobes. In Polygala, one of these lobes
is sterile and sits in the
staminal furrow with the anthers opening above it. This is the pollen spoon. The pollen spoon
catches pollen falling from the anthers, which may dehisce (dry
and open to shed their pollen)
even before the flower opens. The pollen spoon therefore functions
A pollinating insect picks up pollen from the pollen spoon rather
from the anthers.
Note that the flowers of Polygalaceae are monosymmetric: they have only a single
symmetry. Monosymmetric flowers are divided into flag-flowers and
like Milkworts and pea flowers, deliver their pollen to the
underside of the pollinator, as in this
case (they are sternotribic). Lip flowers, as in orchids and
lamiates, attach their pollen to the
back of the pollinator (they are nototribic).
Common Milkwort, Polygala
Note the fruit forming on the branch in the top
right corner. The ovary contains two ovules and is borne on a
short stalk (gynophore). When
fertilised the ovules develop into seeds and the ovary develops
into a green capsule, which
eventually dehisces (dries and splits open) to liberate the seeds.
Each fruit capsule is
(laterally) flattened and enclosed by the two calyx wings which
droop down to enclose it and
lose their petal-like colouration and turn green (below).
Lack (1995) points out the genetic similarities between the
and suggests that, despite the polyploidy
these two species from hybridising, P.
may have evolved from P.
doubling of the chromosomes after failed meiosis. This appears to
be a very common
evolutionary mechanism to generate new species in flowering
Common Milkwort, Polygala
Strawberry Bank, Kent, UK.
the stem of the pink milkwort shown at the very top of this page.
Note how the leaves
are quite elongated, narrow and pointed and become gradually
shorter towards the base of
the stem which lacks a basal rosette of any kind. The most basal
leaves are more rounded /
elliptical. These are characteristic features of Polygala vulgaris. The stems are typically
near the base, since these plants are perrenials.
at the base of the stem of the plant above, which was growing
amongst plants which
were clearly Polygala
at Queendown Warren, we see something quite different as
shown in the pictures below:
a point near the base at which the leaves abruptly change: there
is a loose rosette of
much larger and more rounded leaves. This was followed by a short
section of woody stem at
the very base. A loose rosette of larger leaves part-way up the
stem is a characteristic of
the Chalk Milkwort, which I suspect this plant to be. The majority
examined at this location were of the Polygala
type. Of course, Polygala
variable, and the leaves do become less pointed and more rounded
at the bottom, but a loose
rosette of larger leaves is considered a diagnostic feature of Polygala calcarea. Furthermore,
most of the calyx wing sepals showed only the single pair of
anastomoses also typical of the
Chalk Milkwort (as described by Boswell Syme, 1864). This also
matches specimens held in the
British and Irish Herbarium at the Natural History Museum, London.
However, things are not
quite so simple!
species of Polygala will hybridise when they
co-inhabit the same locality. For example,
crosses between the common and dwarf milkworts, P.
are partially fertile
and grow vigorously (their leaves are said to be bitter-tasting as
do not hybridise at all. What about The Common and
will hybridise with Polygala
however, the offspring are infertile and
cannot pollinate one-another. The offspring do have physical
traits intermediate between teh
two parents, however. The problem is that P.
is tetraploid, meaning it has 4 sets of
chromosomes (4n or 2n = 34 x 2 = 68) two paternal and two
maternal, rather than the usual
two sets (one maternal and one paternal, the diploid 2n state)
is diploid (2n =
34). This means that a gamete from P.
usually contain 2n chromosomes (half the adult
number) and one from P.
n chromosomes. When these fertilise the result is a zygote
with 3n copies of chromosomes, a triploid (3n). Triploids are
often sterile, since the cells have
no strict rule to divide such an odd number of chromosome sets
between the eggs and sperm
during meiosis and production of gametes either fails, or the
gametes are not compatible.
One might think that this rules out the likelihood of finding
plants with a mixture of P.
genes and characteristics. A study by Lack (1995) did reveal a
incidence of Chalk Milkwort genes in populations of Common
Milkwort where the two parents
co-existed either in the present or the past. How can this be?
The answer, Lack suggests, may come from studies on Senecio vulgaris, a tetraploid, which
has acquired genes from the diploid Senecio
(Senecio belongs to the Daisy
Asteraceae). Even though hybrids between these two Senecio species are sterile
do occasionally provide pollen to pollinate Senecio
in a backcross to the parent.
Perhaps they occasionally produce haploid or diploid pollen,
rather than pollen with an
irregular number of chromosomes. In this way, genes from S. squalidus have introgressed into
population, producing plants with some intermediate
Lack suggests that a similar phenomenon has occurred in some Polygala vulgaris populations
acquiring genes from Polygala
Furthermore, Lack employed Trueman's hybrid index,
from Trueman's PhD thesis on some British populations: I don't
have a copy of this
unpublished scale, but I would love to have one!). Lack found that
characteristics in these populations correlated with the presence
of genetic introgression from
P. calcarea. Lack also urges
caution, since P.
may be more variable than generally
Thus, although a population like that at Queendown warren is
mainly Common Milkwort, I would
not rule out the possibility that some of the plants are Chalk
Milkwort or that there has been
some introgression of Chalk Milkwort into this population. The
ultimate test would be genetic
developing fruit: note how the calyx sepals on the lower and older
dropped to envelop the ovaries, but have not yet lost their blue
been recorded that the calyx wings can help dispersal of the whole
fruit capsule by the
wind (the capsule itself also has a small wing or flattened edge).
The capsule contains two
seeds, each of which has a brown/black and hairy seed coat and
bears an elaiosome at one
end (around the hilum, such a protuberance also being called a strophiole). Elaiosomes are
lipid-rich soft structures which act as food for ants and are
there to encourage ants to disperse
the seeds by carrying them off to their nests to eat the
elaiosomes, leaving the seeds to
germinate. Ant dispersal is probably the main mode of seed
dispersal in Polygala. The
elaiosomes are characteristically 3-lobed, with a median lob on
top of the seed and two lateral
lobes extending down the flanks. This has taxonomic value:
and Further Reading
A.J. 1995. Relationships and hybridisation between British species
of Polygala -
evidence from isozymes. New
English Botany, 1864. J. T. Boswell Syme, Mrs Lankester (eds.).
Robert Hardwicke (pub.)
Oostermeijer, J.G.B. 1989. Myrmecochory in Polygala
Forster in a Dutch dune area. Oceologia 78: 302-311.
Stace, C. A. 2010. New Flora of the British Isles. 3rd. ed.
Cambridge: Cambridge University
Note that the strophiole is trilobed. The two lateral lobes extend
down the sides of the
seed to half-way or less in Polygala
to about one-third the length of the seed in
and about one-quarter the length of the seed in Polygala
as nourishment for ants who subsequently take the seeds back to
their nests where they may
germinate. The testa is hard, brown-black and hairy.
Above and below:
Detail of crest.
species of Milkwort do you think is illustrated above? In the
field, what other
characteristics might you look for?
species of Milkwort do you think is shown in the photograph above?
images for full size).