Pea Family (Fabaceae)

Spiny Restharrow

Above:Ononis spinosa (Spiny Restharrow, Upright Restharrow) in late summer

Grows erect to 80 cm (32"). The flowers are usual borne singly at each node of a raceme. (A raceme is an inflorescence with flowers borne single, or in groups, on a pedicel (flower stalk) along a central axis). This plant usually bears (rigid) spines (modified branches, may bear leaves at their base). It is similar in many ways to Ononis repens the Restharrow. Restharrows are perennial herbs or undershrubs with woody rootstocks (rootstock: underground part which can give out new buds). These tough plants with their tough rootstocks made ploughing difficult in the days when ploughs or harrows were pulled by oxen; hence the name 'rest-harrow' since they tended to bring ploughs to a stop! The lower leaves are trifoliate (divided into three leaflets, though the upper leaves tend to be unifoliate (not divided into leaflets). There is a very interesting story about the relationship of these two species, but first let us consider the morphological differences between them. This is particularly important since both occur sympatrically (in the same habitat) in western Europe.

One key difference between O. spinosa and O. repens to aid determination is the nature of the hairs on the stem. Both forms may have short stem hairs, but O. spinosa tends to have two woolly strips of longer non-glandular hairs along the stem, or a single woolly strip which changes sides at each node, giving the appearance of two strips. Descriptions of this character vary and I am not certain that it is reliable. So, what other differences are there? This is something I am currently exploring, in the field/herbarium and in the literature but this is what I have come up with so far:

Ononis spinosa Ononis repens
Stem erect/ascending Stem procumbent then ascending or prostrate
Stem with glandular hairs;
generally hairy all round
Stem generally has fewer glandular hairs;
alternating strip / double strip of longer glandular hairs
Usually bearing long rigid spines, sometimes in pairs Occasionally bearing softer spines in early autumn
Terminal leaflet tip acute; leaflets oblong-oblanceolate Terminal leaflet tip wide and obtuse; leaflets obovate/subrotund
Stem dark reddish Stem greenish, may turn reddish in late summer
Pod equal to or longer than calyx tube Pod shorter than calyx tube
Calyx tube bearing few non-glandular hairs Calyx tube bearing many non-glandular hairs
Diploid: 2n = 2x = 30 chromosomes Tetraploid: 2n = 4x = 60 chromosomes
Habitat: commons, barren pastures, roadsides, heavy clay / limestone soils Pastures, sandy places, coastal cliffs, light well-drained soils

Spiny Restharrow

Spiny Restharrow (Ononis spinosa) in early summer. Note the narrow (acute) leaflet tips and the line of long eglandular (non-glandular) hairs on the stem, which visibly swap sides at the leaf-bearing node. This plant was not particularly upright since it was growing in a heavily grazed area.

Common Restharrow

Common Restharrow (Ononis repens)

This specimen certainly resembles Common Restharrow (Ononis repens) with its flatter and broader leaflet tips and greenish stems. This plant is growing on the same site as Spiny Restharrow's, but looks very different. However, as we shall see things are not quite so simple! To begin with, Ononis repens and Ononis spinosa hbridise. The hybrid form is said to prefer heavier soils, as does O. spinosa and to be morphologically intermediate. Furthermore, the morphologies of both species depend on age of the plant and light levels. This is where things lead to an interesting story. First, let us see how light effects the form of Ononis spinosa:

Morphology of Ononis spinosa growing in full light versus shade:

Full Light Shade
Habit: erect to ascending Decumbent
Spiny from early summer Unarmed (spineless)
Dark reddish stem Greenish stem
Terminal leaflet: narrow, acute Terminal leaflet: wide, obtuse
Full Light Shade
Habit: Decumbent Decumbent
Unarmed (may be spiny in early autumn after flowering) Unarmed (may be spiny in early autumn after flowering)
Greenish stem (may turn reddish in late summer) Greenish stem (may turn reddish in late summer)
Terminal leaflet: wide, obtuse Terminal leaflet: wide, obtuse

Do you notice a pattern here?

O. spinosa growing in shade is more like O. repens in morphology! This has been attributed to a tendency to retain juvenile characteristics: light is required for the plant to reach the full adult morphology with narrower leaflets and lacking spines. Even when growing in full light, very young shoots may have the characteristics of the shade-grown shoots. Some of these features might be adaptive, the reddish stems of plants growing in full light is a common feature of many plants, which redden in response to bright light (and perhaps other stresses) with red pigment helping to protect the chlorophyll from bleaching (the high chlorophyll content of the leaves is possibly masking the red anthocyanin pigment here).

Let us look now at the effects of light on morphology of O. repens:

Not really a pattern here. What can we conclude from all of this? It has been argued that Ononis repens is a neotenic form of Ononis spinosa; that is it evolved from O. spinosa by a process of neoteny. This is the phenomenon whereby an organism (it is usually applied to animals) evolve by retention of juvenile characteristics in the adult. Human beings are an example of this: they have several features of juvenile or embryonic primates retained in the adult, including the upright posture and large brain to body mass, lack of hair (and several other features). However, note that O. repens has twice as many chromosomes as O. spinosa: it evolved by a doubling in chromosome number. It has been pointed out that polyploidy (an increase in the number of sets of chromosomes) in flowering planst may often slow growth in plants and delay flowering. Neoteny, whereby the plant flowers before acquiring the full adult form may hasten this, allowing reproduction to be affected in a timely manner despite polyploidy. In the shade, where maturation is held back, and in very young shoots, the morphology is more like that of O. repens. In contrast, older shoots of O. repens in autumn begin to resemble more the mature shoots of O. spinosa. This does, however, complicate determination for the field or herbarium botanist.

Spiny Restharrow

Above: Spiny Restharrow (Ononis spinosa)

Which species are shown in the two photographs below?




Flowers of the Pea Family characteristically have five petals with the uppermost often upright and advertising the flower to pollinators - this top petal is called the standard (flag). at the sides are a pair of wing petals and underneath are a pair of keel petals which often form a boat-shaped structure. These petals are more or less tightly interlocked at their bases by interlocking folds or projections or adherent cells; the exact mechanism depending on species.


The flowers of Fabaceae are evidently zygomorphic (with bilateral symmetry, that is symmetric left and right halves but differing from top to bottom).



Hippocrepis comosa (Horseshoe Vetch)


Although the flowers of Horseshoe Vetch are arranged in a complete circle or incomplete arc of 5 to 8 (to 12) flowers at the top of the peduncle (inflorescence stem), the name derives from the fruit: pods which are beautifully constructed from horseshoe-shaped segments (like a chain of crescent moons) which curves downwards. The front of each crescent 'horseshoe' bears rough points. The articulated pod breaks down into one-seeded crescent-shaped segments.

Horseshoe Vetch is perennial, with a woody rootstock, and reaches 20 cm in height. It puts out a number decumbent (lying-down) branching stems 15 to 45 cm in length which put up numerous inflorescences. Each inflorescence consists of an upright stalk or peduncle topped by a whorl of 5 - 8 (-12) flowers, each flower borne on a short stalk or pedicel and beneath the whorl is a very short involucre or collar of scarious (dry and membranous) bracts. note that the petals have long claws (the narrow basal part of the petals) as long as the blade or  lamina of the petal.


Above: a single peduncle of a Horseshoe Vetch. The bell-shaped (campanulate) calyx tube of 5 sepals consists of a united pair uppermost which are separated from the 3 lower sepals. The pedicels are shorter than the calyx tube and the petals more than three times as long as the sepals. It is possible to confuse Horeshoe Vetch with Lotus (see below) but the pinnate leaves of Horseshoe Vetch are divided into more leaflets (usually 4 to 7 pairs of leaflets plus a terminal leaflet).

Hippocrepis comosa is a European species, occurring naturally from Spain eastwards to Greece and Bulgaria and from northern England southwards to Sardinia and Sicily. It occurs as a diploid, a tetraploid and a hexaploid form, which are morphologically almost indistinguishable. Fearn (1972) studied the distribution of these chromosomal types and found that in France and England, the diploid race was scattered among the tetraploid race, with the hexaploid occurring in the Pyrenees. furthermore, the diploid was found primarily on limestone, such as cliffs and rocky slopes, whilst the tetraploid was more widely distributed but occurred mostly on grazed or formerly grazed chalk downs. Fearn suggested that this difference in distribution could be explained thus: the prostrate tetraploid form was more resistant to grazing and could reproduce asexually by layering, further increasing resistance to grazing. in contrast teh diploid form reproduced only by seed, making it vulnerable to grazing.



Fearn, G.M. 1972. The distribution of intraspecific chromosome races of Hippocrepis comosa L. and their phytogeographical significance. New Phytol. 72: 1221-1225.

Lotus (Bird's-foot Trefoil)

Lotus, bird's-foot-trefoil

Above: Bird's-Foot Trefoil (Lotus) or Birdsfoot Trefoil: see also Meadow Flowers

The name 'trefoil' refers to the leaves which appear trifoliate (divided into 3 leaflets) but there is an additional pair of leaflets at the base of the petiole which are often narrower and appear like stipules, but the true stipules are tiny brown scales that are hard to see. The leaflets have entire or minutely serrate margins.

Birsdfoot Trefoil occurs naturally in Europe, North Africa and parts of West Asia, but has been introduced  in parts of America, Iceland, South Africa and Australia (external link: see distribution map). It is a nutrient-rich plant cultivated for fodder.

Birdsfoot trefoil is perennial with a deep woody taproot up to 1 m long which gives out numerous lateral roots and may produce dense fibrous roots in the topsoil.The rootstock puts out a few subterranean stolons, numerous stems spread outwards in a circle.

The prostrate stems lie on or just beneath the soil surface and are rooting. they are prostrate (lying down) or decumbent (ascending at the tips). The inflorescence stalks (peduncles) emerge from leaf axils and have 2 to 6 flowers each. Each flower is borne on a short pedicel (1 to 8 mm in length).

Lotus, bird's-foot-trefoil

Above and below: Lotus corniculatus (Common Birdsfoot Trefoil) can be identified by the following characteristics:

The sepal tube (calyx) is formed of five united sepals and has five teeth (free sepal tips), the two uppermost of which have an obtuse angle between them (see below). The teeth are upright in bud (not curved backwards/outwards as in Lotus pedunculatus, the Greater Birdsfoot Trefoil). The flattened flower buds are a dark red color. The stems are low and prostrate and solid (they are upright and partly hollow in the introduced cultivated variety var. sativus, which also has all yellow flowers without orange or red tinges).

It can be confused with Lotus tenuis (= Lotus glaber, Narrow-leaved Birdsfoot Trefoil) but the latter has narrower (linear to lanceolate) leaflets, especially near the stem apex. L. tenuis is found on dry grassland, preferring heavy clay soils and is diploid (2n = 12). Lotus pedunculatus (Greater Birdsfoot Trefoil)  is also similar but in bud its sepal teeth curve backwards and the angle between the two topmost sepals is acute not obtuse. L. pedunculatus is found in damp places, such as marshes and fens.

Lotus, bird's-foot-trefoil

There is a bract divided into three lobes (trifoliate) just beneath the flowers and just visible above.

Pollination Mechanism

There are different color varieties of Lotus corniculatus, under genetic control, with petals varying from pure yellow to orange. The standard may have reddish lines. Pollinating bees often show no apparent preference, with the possible exception of Bombus lapidarius which is said to prefer the darker flowers.

The 10 stamens form a stamen tube in which 9 stamens are united, but the 10th and uppermost stamen is free. Nectar is secreted between the base of the stamen tube and the ovary it encloses, so the stamen tube collects the nectar. Bombus can use its long proboscis to reach the nectar.

Bumblebees (Bombus) use the keel and wings as a landing platform. The weight of the landed insect depresses the wings and keel forcing the stamen tube beneath to act as a piston, forcing out pollen onto the underside of the insect. (Zygomorphic flowers that place their pollen on the underside of the insect are called sternotribic, as opposed to nototribic flowers that place their pollen on the upper-side of the insect, such as members of the deadnettle family and orchids). When receptive, the stigma will protrude through a slit at the tip of the keel (the edges of the keel petals are fused together except for a small slit in the tip) to receive pollen. When teh insect leaves, the wings return to their default position within about 15 minutes. Lotus corniculatus is usually cross-pollinated, but it is thought that self-pollination may sometimes occur. This mechanism of pollination is similar in Hippocrepis comosa and Ononis.

Lotus, bird's-foot-trefoil

Fruit: The cylindrical seed pods are 15 to 30 mm long and held out horizontally from the peduncle (inflorescence stalk). Collectively the group of pods on each peduncle resemble a bird's foot! the pods are brown to black when ripe. Each seed is little more than 1 mm in length.

Lotus, bird's-foot-trefoil

The fruit develop around June onwards and by late September new creeping stems and buds develop below ground and short shoots may also occur above ground throughout the winter.

Lotus, bird's-foot-trefoil

Lotus corniculata is a very variable plant and found in a wide variety of habitats: grasslands, heaths, cliffs, roadside verges and is a pioneer species of quarries, from coastal to alpine. It is tetraploid (4x = 2n = 24). It avoids aqueous and very humid habitats, shade and very acidic soils. Populations at lower elevation tend to have a semi-prostrate habit with more stems and flower later. Apart from var. corniculatus, other varieties include: hirsutus, crassifolius, norvegicus and sativus. 'Hirsutus' = hairy; 'crassifolius' = thick-leaved (leaves succulent); 'norvegicus' = 'native to Norway'; 'sativus' = cultivated. Some genetic types are cyanogenic: the leaves and petals being capable of generating cyanide as a defense against grazing molluscs.

References for Lotus:

Jones, D.A. and Turkington, R. 1986. Lotus corniculatus L. Journal of Ecology  74(4): 1185-1212. DOI: 10.2307/2260243

Compton, S.G., Beesley, S.G. and Jones, D.A. 1988. Variation in the colour of the keel petals in Lotus corniculatus L. 5. Successional differences in the distribution of dark-keeled plants. Heredity 61: 235-245.

Article updated: 29 June 2020.