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