Broomrapes

Orobanchaceae (Broomrapes)

Above and below: Orobanche minor var. minor (Common Broomrape). This species occurs in many parts of Europe and northern and eastern Africa but has been introduced to parts of the American continent, southern Africa and other parts of Europe.

Broomrapes are parasitic plants. They are holoparasites, meaning they are entirely dependent on their host plant for nutrition. They are root parasites - forming junctions called haustoria between themselves and the roots of their host through which the broomrape diverts nutrients from the host into itself by diverting the flow of host phloem sap. In many members of the family, haustoria are borne at the end of fine roots, but in broomrapes (Orobanche) the base of the stem (often swollen) is joined directly to the host root.

Orobanche minor is a complex group of races with various host preferences.This variety parasitises Asteraceae and Fabaceae. Fabaceae (Pea Family) are evident near the bases of the Orobanche minor in these photos - namely Trifolium.

This variety occurs on stable dunes, shingle, roadside, chalk and gravel pits, etc. and is an annual.

Orobanche minor var. minor stems reach up to 60 cm in height (but is usually less) are yellowish to reddish-brown, tinged purple, and typically covered in fine hairs. Each flower is subtended by a bract which is more-or-less the same length as the corolla (petal tube) or slightly longer. The corolla is made up of 5 petals, 2 fused to form an upper lip and 3 fused to form the lower lip. The corolla is marcescent - retained when dead. The stigma has pink-purple-red lobes. These can be seen in these photos just inside the corolla. Yellow forms lacking anthocyanin have yellow stigma lobes.

There are 4 stamens, 2 with long filaments and 2 with shorter filaments (the didynamous condition - a characteristic of the family). The filaments are attached to the inside of the corolla tube, just below mid-way. The anthers split by longitudinal slits and each consist of two pollen chambers or lobes, each with a elongated spur or mucron (said to be mucronate). The fruit is a single-chambered capsule, splitting into 2 valves when dry, releasing many minute seeds.

The flowers of the Orobanchaceae have ancestral pentamerous symmetry as can be seen in the petal lobes, but one of the stamens is either absent or reduced, as is typically one of the sepal lobes.

Orobanche minor is notoriously variable. However, some general features are as follows (see Thorogood and Rumsey, Broomrapes of Britain & Ireland, 2021; BSBI handbook 22): pinkish-purple to reddish-purple stigma lobes (yellow in all yellow variants) ; bracts as long as or longer than corolla (petal tube); corolla tube usually curved throughout and creamy-yellow variously flushed with purple; upper lipemraginate, sometimes weakly bilobed; lower lip with 3 lobes, middle lobe largest with a crisp appearance and toothed margin.

Orobanche minor has a large host range, but favors Asteraceae and Fabaceae. These specimens were growing among Red Clover (Trifolium pratense, Fabaceae).

Below: Orobanche hederae (Ivy Broomrape) identifiable by the yellowish stigma lobes and the constriction of the corolla just behind the corolla mouth. This form flowers into autumn and is a native of western Europe, the Mediterranean and northern Iran. It is a usually a monocarpic biennial, flowering once in the second year (sometimes later) before dying.

Below: Orobanche picridis (Picris Broomrape) is parasitic mainly on Picris hieracioides (Hawkweed Ox-tongue). It is the rarest broomrape native to the British Isles. It is an annual.

The corolla is pale ivory in color and the stigma lobes are dark purple and just touching, or almost black in some populations. The combination is striking and beautiful. The flowers are usually confined to the top half of the stem.

There are some 90 genera and over 2000 known species of Orobanchaceae. Most of these genera are parasitic (either holoparasites or plants only partially or optionally dependent on their host for nutrition - hemiparasites).

Lathraea (Toothworts)

Above and below: Lathraea squamaria (Toothwort) is naturally found in Europe and parts of Asia. It is parasitic on the roots of hardwood trees, especially Hazel (Corylus avellana). Like Orobanche, Lathraea is not photosynthetic. Toothwort is perennial, persisting as a branched subterranean rhizome which puts out fibrous roots that attach to host roots by haustoria (minute tubercles). For details on the biology of Toothwort see: Parasitic Plants.

The shoots are non-photosynthetic, appearing above ground only for flowering and disintegrating to disperse their fruit. The stigma is 2-lobedand projecting; typical of the family, there are 4 stamens, 2 with long filaments and 2 with shorter filaments (the didynamous condition).

A number of genera of hemiparasitic plants - plants that depend partially or optionally upon their host and which are capable of photosynthesis and which were lumped together in the family Scrophulariaceae have recently been moved into the Orobanchaceae. These include the Cow-wheats, Bartsias, Hay-rattles and Eyebrights.

Melampyrum (Cow-Wheats)

Above and below: Melampyrum pratense (Common Cow-wheat) var. montanum. The bracts (modified leaves on the flowering stem, particularly when immediately beneath the flowers) are green and lanceolate (lance-head shaped) to ovate in contour and pointed (acute). In var. montanum the bracts lack prominent teeth. (The calyx (sepal tube) is divided into 4 teeth which can be seen curving backwards). This variety is found in heaths and pastures but was growing here beside a road on the edge of woodland which is more the preferred habitat of var. vulgaris, the common form but in vulgaris the bracts are strongly toothed. The corolla is pale yellow with a whitish tube (which is particularly white in var. montanum). The flowers are borne on a lax raceme (a monopodial axis with flowers borne on stalks). The 4 stamens are didynamous.

Common Cow-wheat is an annual found naturally in Europe and northwest Asia. The fruit is a compressed capsule splitting when dry into two valves. The plant derives its common name from its large seeds which resemble wheat but were considered good only for cows. Indeed, cows able to feed on Cow-wheat were said to produce especially rich butter.

Odontites (Bartsias)

Above and below: Red Bartsia - Odontites verna var. vulgaris (Odontites verna is the accepted name but Odontites vernus is often used). Red Bartsia is an annual.The branches are opposite and erect or (spreading and curving upwards in var. serotina). The bases of the leaves are rounded in var. vulgaris, narrowed towards the base in var. serotina.

Red Bartsia is a hemiparasite of grassland plants such as White Clover (Trifolium repens) and certain grasses such as Barley (Hordeum vulgare). It's roots attach to the roots of its host by haustoria in which the connection consists of a few tracheids tapping into the host's xylem. It assimilates small sugars (monosaccharides) and organic nitrogen (amides, amino acids) and organic acids from its host. Leaf glands secrete water (guttation) which presumably maintains this nutrient supply even when the host is not transpiring rapidly.

The corolla (petal tube) is dull pink and tubular or bell-shaped. The upper lip is more-or-less entire and the lower lip is divided into 3 lobes. The calyx is partially divided into 4 teeth. There are 4 stamens, 2 on long filaments and 2 on shorter filaments (i.e. the stamens are didynamous) and positioned beneath the upper lip. Each anther has two pollen-producing compartments, forming two pointed lobes. The fruit is a capsule that opens by two valves and contains many seeds of various sizes.

Red Bartsia is Eurasian and native to Europe, Iran and the west Himalayas but is naturalized elsewhere.

Rhinanthus (Hay-Rattles)

Above and below: Rhinanthus minor (Common Yellow-Rattle) - an annual hemiparasite found in meadows, damp pastures and marshes which parasitises grasses. Note the form of the flowers: 'rhinanthus' literally means 'nose flower'. The erect stems bear leaves in opposite pairs and are deeply serrated.

The upper lip of the corolla is helmet-shaped and laterally compressed and has 2 small violet teeth at its apex. The lower lip is three-lobed. The 4 stamens are didynamous and held under the upper lip. The anthers have two pollen chambers / lobes which are not mucronate.

Below: the name 'Hay-rattle' derives from the fact that the seeds rattle inside the fruit capsules.

Euphrasia (Eyebrights)

See also: Euphrasia (www.cronodon.com/NatureTech/euphrasia.html)

Above and below: Euphrasia nemorosa (Common Eyebright) an annual hemiparasitic on a range of host plants of meadows, heaths and pastures. It occurs naturally in Europe but also introduced into parts of North America. The 4 stamens are again didynamous and the mucron of the inner lobe on the shorter pair is elongated to an awn.

Acknowledgments

Thanks to Dr Fred Rumsey of the Natural History Museum, London, with help determining Broomrape (Orobanche) taxa.

References

Rumsey, F.J. and Jury, S.L. 19991. An account of Orobanche L. in Britain and Ireland. Watsonia 18: 257-295.

Thorogood, C.J. and Rumsey, F. 2020. An account of Common Broomrape Orobanche minor (Orobanchaceae) in the British Isles. British & Irish Botany 2(3): 223-239.

Thorogood, C. and Rumsey, F. 2021. Broomrapes of Britain & Ireland BSBI Handbook 22. Botanical Society of Britain & Ireland.