Polygalaceae (Milkwort Family)

Above: Polygala vulgaris, the Common Milkwort (Queendown Warren, Kent, UK). These are small flowers, the calyx wings of this species are 6 to 15 mm long.

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 calcarea, the Chalk Milkwort, and Polygala vulgaris, the Common Milkwort. The heath Milkwort, Polygala serpyllifolia, is also common in the British Isles, but has not been recorded from the habitats examined here. Polygala amarella (Dwarf Milkwort, formerly Polygala austriaca) is also found in the British isles, but is restricted to a very few localities.

Above: Polygala vulgaris.

Above: Polygala sp., Queendown Warren, Kent, UK.

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 wings, formed by the inner lateral sepals which resemble petals (petaloid sepals) and are therefore not equivalent to the wings in pea flowers. These calyx wings form 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 (sides) of 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 platform 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 oxyptera).

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 nectar guide flags. 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.

Polygalaceae have a pentamerous ancestor (a flower with whorls of five appendages) and 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 Bello et al. (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 chamber.

Above: a 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 pollinator.

Above: 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 of auricles and crest; 4. Stamens and pollen spoon; 5. Androecium (stamens); 6 sepals enclosing fruit capsule; 7. Fruit capsule (note the small wing).

The 8 stamens of Polygalaceae have their filaments united to form a kind of cup or staminal furrow 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 terminal pore as illustrated below:

The 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 in secondary pollen presentation. A pollinating insect picks up pollen from the pollen spoon rather than directly from the anthers.

Note that the flowers of Polygalaceae are monosymmetric: they have only a single plane of symmetry. Monosymmetric flowers are divided into flag-flowers and lip-flowers. Flag-flowers, 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).

Above: a 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 example.

Above: Common Milkwort, Polygala vulgaris, Yocklett's Bank, Kent, UK. The flowers seen here
have short crests.

Distinguishing species of Polygala

The picture above and the picture below show Polygala vulgaris, 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.

Having 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 vulgaris subsp. oxyptera. The fourth pattern is characteristic of Polygala calcarea, the Chalk Milkweed: There is a single pair of anastomoses between the 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 serpyllifolia (Polygala depressa) 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. vulgaris and P. calcarea by their much smaller flowers (and P. amarella has a well formed and compact rosette of basal leaves near where the shoot 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.

Above: Common Milkwort, Polygala vulgaris. 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).

It has 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:

Above: Note that the strophiole is trilobed. The two lateral lobes extend down the sides of the seed to half-way or less in Polygala calcarea, to about one-third the length of the seed in Polygala vulgaris and about one-quarter the length of the seed in Polygala amarella. It serves 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: developing fruit: note how the calyx sepals on the lower and older flowers have dropped to envelop the ovaries, but have not yet lost their blue colour.

Above: Common Milkwort, Polygala vulgaris, Strawberry Bank, Kent, UK.

Above and below: Detail of crest.

Above: 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 woody near the base, since these plants are perrenials.

Looking at the base of the stem of the plant above, which was growing amongst plants which were clearly Polygala vulgaris at Queendown Warren, we see something quite different as shown in the pictures below:

There is 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 Polygala calcarea, the Chalk Milkwort, which I suspect this plant to be. The majority of plants examined at this location were of the Polygala vulgaris type. Of course, Polygala vulgaris is 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!

Polygala Hybrids

Certain species of Polygala will hybridise when they co-inhabit the same locality. For example, crosses between the common and dwarf milkworts, P. vulgaris X P. amarella, are partially fertile
and grow vigorously (their leaves are said to be bitter-tasting as in Polygala amarella). However, P. vulgaris and P. amarella do not hybridise at all. What about The Common and Chalk Milkworts?

Polygala vulgaris will hybridise with Polygala calcarea, however, the offspring are infertile and cannot pollinate one-another. The offspring do have physical traits intermediate between the two parents, however. The problem is that P. vulgaris 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) whilst P. calcarea is diploid (2n = 34). This means that a gamete from P. vulgaris usually contain 2n chromosomes (half the adult
number) and one from P. calcarea 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. vulgaris
and P. calcarea genes and characteristics. A study by Lack (1995) did reveal a significant 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 squalidus (Senecio belongs to the Daisy family, Asteraceae). Even though hybrids between these two Senecio species are sterile triploids, they do occasionally provide pollen to pollinate Senecio vulgaris, 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 the S. vulgaris population, producing plants with some intermediate characteristics.

Lack suggests that a similar phenomenon has occurred in some Polygala vulgaris populations
acquiring genes from Polygala calcarea. 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 assessing intermediate characteristics in these populations correlated with the presence of genetic introgression from P. calcarea. Lack also urges caution, since P. vulgaris may be more variable than generally assumed.

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 testing.

Common Milkwort, Polygala vulgaris subsp. vulgaris.

Lack (1995) points out the genetic similarities between the diploid Polygala serpyllifolia and the tetraploid Polygala vulgaris and suggests that, despite the polyploidy barrier preventing these two species from hybridising, P. vulgaris may have evolved from P. serpyllifolia by a doubling of the chromosomes after failed meiosis. This appears to be a very common evolutionary mechanism to generate new species in flowering plants.

Q. Which species of Milkwort do you think is illustrated above? In the field, what other characteristics might you look for?

Q. What species of Milkwort do you think is shown in the photograph above? (Click images for full size).

References and Further Reading

Lack, A.J. 1995. Relationships and hybridisation between British species of Polygala - evidence from isozymes. New Phytol. 130: 217-223.

English Botany, 1864. J. T. Boswell Syme, Mrs Lankester (eds.). Robert Hardwicke (pub.) London.

Oostermeijer, J.G.B. 1989. Myrmecochory in Polygala vulgaris L., Luzula campestris (L.) DC. and  Viola curtisii 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 Press.

Article updated: 24 April 2022.