Primulaceae (Primrose Family)
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Primrose (Primula vulgaris)
Primula vulgaris (Common Primrose) is primarily a woodland plant of southern and western Europe, preferring shade that is not too heavy and a reasonable level of dampness. In damp conditions it can also be found in more open hedgerows and occasionally meadows. It thrives wherever there is a breach in the canopy, but persists once mature for quite some time in heavier shade. It is also found in the Alps and on the southern slopes of the Carpathians.
Primula vulgaris is a rosette-forming perennial. It flowers early, primarily from March to May, and forming a rosette makes the plant more resistant to snow and frost.
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| Pin-eyed form | Thrum-eyed form |
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Most
Primroses, including Primula vulgaris,
have an interesting mechanism to reduce self- pollination and
enhance cross-pollination, meaning that they are designed to be
pollinated most easily by pollen from a different plant.
Cross-pollination prevents 'in-breeding' and maintains genetic
diversity in a population, which also helps to counteract the
effects of detrimental mutations in genes, since with
cross-breeding, an organism is more likely to inherit a healthy copy
of a gene if its parent population contains faulty copies. Primrose
flowers, even on the same plant, come in two morphological
varieties: pin-eyed and thrum-eyed. Pin-eyed are so-called
because the carpel has a long style that raises the stigma to the
top of the floral tube, where it is visible from above as a
'pin-eye' like structure. In a pin-eyed
flower
the stamens are lower down than the stigma in the floral tube. In thrum-eyed flowers, the style is shorter and
the stamens higher, so that the stigma is far beneath the stamens.
Pollen must travel from a stamen to a stigma, and attach to the
stigma, to effect pollination.
If an insect with a long proboscis visits a pin-eye flower and
reaches for the nectar at the base of the floral tube, then pollen
will rub onto its proboscis. If this same insect then visits a
thrum- eye flower, then the pollen is at the right height to rub off
onto the short stigma of the thrum- eye flower. The insect's head
will also become covered with pollen from the thrum-eye flower. If
this insect now returns to a pin-eye flower then the pollen on its
head will be at the correct height to rub off onto the stigma of the
pin-eye flower.
The form a particular flower develops into is under genetic control.
Effectively, plants with at least one dominant S allele (an allele
is a variant of a gene inherited from either parent) develop into
the thrum-eyed form, whereas those with two recessive s alleles
develop into the pin-eyed form. However, some British populations
have a third form, the homotypic form which has the high anthers of
the thrum-eyed form and the high stigma of the pin-eyed form, such
that the stigma and anthers are at the same height. This form is
self-compatible and can therefore self-pollinate with high
efficiency. Consequently it may produce ample seed in the absence of
pollinators, but may lose out to the thrum-eyed form when suitable
pollinators are abundant (see the study by: Boyd et
al.
2003. Population ecology of heterostyle and homostyle Primula vulgaris: growth, survival and
reproduction in field populations. J.
Ecol.
78: 799-813. The scent of Primula
vulgaris
is strongest at night and the main pollinators are thought to be
nocturnal moths. The normal pin and thrum-eyed type (the heterostyle
type) is perhaps at a disadvantage in small populations, however,
where a suitable pin or thrum partner must be found by the
pollinator. Indeed, in Belgium the plant is rare and in decline and
this may be due to habitat fragmentation (Brys et
al.
2004. Reduced reproductive success in small populations of the
self-incompatible Primula
vulgaris.
J. Ecol. 92: 5-14).
The
Primrose belongs to the family Primulaceae which has the following
general floral formula:
* K5 [C(5)
S5+A5]G(5)
Where the anthers consist of five fertile anthers alternating with 5
sterile anthers or staminodes (S), the filaments (anther stalks) of
which are fused to the corolla tube which itself consists of 5 fused
petals. The female gynoecium (ovary) is inferior and formed from 5
fused ovaries. There are many ovules within the gynoecium, arranged
around a central axis to which each ovule is joined by its placenta
(central placentation). There is a nectary at the base of the
gynoecium.
Above: Primula vulgaris flowering in February during one mild winter; growing in a wood of predominantly beech mixed with ash, hornbeam and some oak.
Pin-eyed or thrum-eyed?
Cowslip
(Primula
veris)
The Cowslip is another perennial. Whereas the flowers of the Common
Primrose are usually borne singly on pedicels (up to 15 cm tall)
arising from the leaf rosette and only rarely clustered in an umbel
on a stalk or scape, the flowers of cowslip are clustered in an
umbel borne on a scape up to 30 cm tall.
The
Cowslip should not be confused with the Oxslip (Primula
elatior).
Hybrids form between
Primula
vulgaris, veris
and elatior, adding to possible
confusion. The Cowslip is found on grassland.
Scarlet Pimpernel Anagallis arvensis subspecies arvensis
The Scarlet Pimpernel is usually annual and the stems, up to 40 cm long, can be lie on the ground but may curve upwards (ascending) sometimes only at the tip (decumbent). The petals are often have tiny teeth at their margin (they are denticulate) and bear many tiny 3-celled glandular hairs. This plant is found on open grasslands and dunes and flowers from June to August.
Instantly recognizable by its solitary small red flowers with pink/purple central 'eyes', Scarlet Pimpernel is also known as Poor-Man's Weatherglass due to it closing its flowers in dull, overcast or damp weather. In fair weather the flowers generally open at 8:00 am and close at around 2:00 pm to 4:00 pm. There are 5 sepals, 5 (deciduous) petals and 5 stamens (with free filaments), the flower described as rotate (flat like a wheel) but may be slightly concave or rarely funnel-shaped. Each flower originates from an axillary bud (in the axil of a leaf) and borne on a peduncle (inflorescence stalk) which becomes recurved in fruit. the petals have obtuse (blunt) tips with tiny teeth towards the apex. The sepals are lanceolate (narrow) with acute tips and membranous borders. The fruit is a globular capsule opening tranversely with the top part detaching like a lid to release numerous seeds.
Phyllotaxy (arrangement of leaves):In all young shoots the leaves are decussate, that is one pair per node with the pair of leaves at adjacent nodes alternating by 90 degrees. However, a detailed analysis by Kwiatkowska (1995) (Ontogenetic changes of phyllotaxis in Anagallis arvensis L.) showed that, rather unusually, the phyllotaxy may change several times along a growing shoot. the leaves are sometimes arranged in a trimerous (3 leaves per node) or tetramerous (4 leaves per node) arrangement with the leaves in true whorls. More typically, however, nodes with 3 or 4 leaves are pseudowhorls (due to very short nodes in between each pair of leaves) with spiral phyllotaxis. In those shoots with pseudowhorls of 3, the spiral taxis follows the Fibonacci series with an angle of 137 degrees between successive leaves (very close to the ideal Golden Ratio). Shoots, or segments of shoots, with 4 leaves per node usually follow Lucas spiral phyllotaxy, in which the angles between successive leaves are determined not by the Fibonacci series, but by the related Lucas series (2, 1, 3, 4, 7, 11, 18, 29, 47, 76, 123, ...). Both the Lucas and Fibonacci series eventually converge on the same Golden Ratio. Switches from decussate to spiral phyllotaxy occur in a number of plant species, but unusually in Anagallis arvensis the phyllotaxy may change several times along a single shoot, starting at decussate but switching between the various pseudowhroled and whorled phyllotaxies sometimes more than once.
Above: the globose fruit capsule of Red Pimpernel is of a type called a pyxide (pyxis or pyxidium - a capsule whose upper part lifts up as a lid). It is 3 to 4 mm in diameter. The capsule dehisces along the equatorial line and the lid (operculum) detaches. (Drawing based on Drobnik and Bacler, 2007 who first ellucidated the detailed structure and function of the fruit: A new way of dissemination in Anagallis arvensis L. (Primulaceae). Acta Societatis Botanicorum Poloniae 76(3): 251-253). The lid bears the persistent style but the deciduous petals have fallen, leaving the pointed sepals at the base of the capsule. There are ten vascular bundles visible in the wall of the capsule.there is a single cavity within the fruit (the ovary is unilocular).
Inside the thin pericarp (capsule wall) is a central 'columella' which is actually globose and shrinks slightly as it dries, separating from the pericarp. It remains attached inside the base of the capsule but is easily detached and drops out (the peduncle at this stage is recurved and rigid, so the capsule is hanging downwards). Its surface is covered with pits, each enclosing a single seed. There are 20 to 30 seeds per capsule / columella and each plant may produce 900 to 1200 seeds in total (i.e. up to about 50 or so fruit). The columella rolls along the ground, shedding some of the seeds which protrude slightly above the surface due to the shrinkage of the columella. Each brownish-black seed, about 1 mm or less in diameter, is a frustrum (truncated cone) forming a truncated pentagonal or hexagonal pyramid shape with rounded lateral edges. The wide end sits at the surface of the fruit. This shape clearly facilitates falling out of the seeds as the columella rolls along. A very short funiculus connects each side to the bottom of the pit (this severs when the seed is ripe).
Above and below: Anagallis arvensis in fruit, showing pyxidia. The peduncles are clearly recurved.
The Yellow Pimpernel is a perennial whose stems may grow erect or lie prostrate and reach
40 cm in length and may be ascending towards their tips (decumbent).
Article updated: 26 Aug 2017, 12 Aug 2020, 14 Aug 2020, 21 Sep 2020, 2 May 2022