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Saturday, February 19, 2011

Primrose, Primula vulgaris, Primulaceae

With the possible exception of the bluebell, the primrose Primula vulgaris is probably Britain's most popular spring flower. Even now, on a freezing mid-February day, new leaves and flower buds are beginning to form in primroses in woodlands here in north-east England.  Primroses have been grown in gardens for centuries - probably since people first cultivated gardens - giving rise to numerous varieties and, through hybridisation with cowslips, the garden polyanthus. The primrose also excited Charles Darwin's curiosity, in his struggle to understand and define the nature of species. Like many before him, he was aware that ...

........ primroses, with their large single flowers, grew along woodland edges and hedgebanks....

... while cowslips, with their numerous small flowers on a common stalk, grew in pastures, but...

... wherever the two coincided they hybridised to produce false oxlips, with large flowers on a common stalk. Although every field guide to wild flowers describes primroses and cowslips as distinct species, in evolutionary terms they are really one - a genetically diverse species with a wide range of variation which, at its extremes, produces distinctive plants that are adapted to life in woodlands or grasslands. Primroses and cowslips are on the way to becoming two separate species, where they would satisfy the modern evolutionary biologist's absolute definition of a species, based on an inability to interbreed with other related species, but they haven't reached that point yet. No wonder this example of evolution-in-progress attracted Darwin's inquiring eye.

Ever since primroses and the primrose-cowslip false oxlips hybrids were introduced into gardens they have been exchanging their genes with other cultivated Primula species from the European mainland, introducing new flower colour genes that give us the range of brilliant hues that are available in garden centres today. Sometimes the gene exchange extends beyond the garden and back into the wild - as, for example, in the primrose you can see here.

Darwin was fascinated by primroses for another reason. His Cambridge University botany teacher and mentor, John Stevens Henslow, drew his attention to the fact that there are two kinds of flowers in any wild population of primroses... pin-eyed flowers, with the stigma at the end of a long style, level with the top of the corolla tube...

... and with the stamens located way down in the corolla tube, as you can see in this sectioned flower...

... and thrum-eyed flowers, like this, where the stamens are at the top of the corolla tube...

... and where the stigma, at the end of a much shorter style, is located way down in the corolla tube.

The presence of two forms of flowers in the population is known as heterostyly and Darwin made numerous self- and cross-pollinations between the two forms, demonstrating that self-pollination failed and that crossing beween the two forms was necessary for seed set, and also that the differential placement of the stigma and stamens in the two forms aided cross pollination between them by insects. In his autobiography he remarked "No little discovery of mine ever gave me so much pleasure as the making out of the meaning of heterostyled flowers"

The two forms were illustrated in diagrammatic form in this publication: Darwin, C. R. 1862. On the two forms, or dimorphic condition, in the species of Primula, and on their remarkable sexual relations. [Read 21 November 1861] Journal of the Proceedings of the Linnean Society of London (Botany) 6: 77-96.

The scientific study of this floral arrangement has yet to run its course and, 150 years after Darwin first described the mechanism, the genes that control the development of the two primrose flower types are currently under investigation by Professor Phil Gilmartin at Durham University.

Darwin wasn't the only eminent Victorian to be beguiled by this flower. Primroses were the favourite flower of Benjamin Disraeli, Prime Minister in two Conservative governments during Queen Victoria's reign. Such was her affection for him that she sent a wreath of primroses to his funeral on 19th. April 1881. Thereafter that date was celebrated annually as Primrose Day.

Wednesday, February 2, 2011

Passion Flower, Passiflora citrina, Passifloraceae

Passion flowers Passiflora spp. are amongst the most elaborate of all flowers but I particularly like the relative simplicity of this diminutive species, Passiflora citrina which hails from the hills of western Honduras and eastern Guatemala. Its mountain origins mean that it does well in a cool conservatory. The flowers are only a few centimetres long and the whole plant will happily grow in a large pot on a windowsill, trained up slender canes, where it will flower for months on end.

The structure of Passiflora blooms is unusual, with the ovary and stamens held aloft from the centre of the flower on a long column. It has been suggested that this arrangement evolved to avoid damage from visiting pollinators (usually bees, although large red-flowered species are visited by hummingbirds) that probe for nectar at the base of the petals. Keeping the ovary - all important for seed production - out of harm's way will making sure that the visiting pollinator contacts those stamens and stigmas on the way in and out is a win-win strategy from the plant's perspective. The flowers are protandrous, meaning that the anthers shed their pollen first and then, once it has all been removed by pollinators, those three nail-shaped stigmas on top of the ovary move outwards and become receptive, so promoting cross pollination. 

Passion flowers are loaded with religious symbolism, with the number of floral parts being taken by the faithful to symbolised various episodes in the life of Christ. For example, the ring of floral filaments is considered to represent the crown of thorns, the three stamens represent the nails used in the crucifixion and the five stamens represent the five wounds - usually with this species Passiflora caerulea being the 'typical' symbolic passion flower.

The edible passion flower P.edulis, like the one above, is easy to grow from the dimpled seeds in commercial passion fruits, although the flowers are quite small and lack the boldness and symmetry of P.caerulea. I've found that the best way to germinate these seeds, and those of many other tropical and subtropical plants like lychees, is to clean the seeds then put them in a polythene bag of moist peat (or a substitute like coir) and keep them in the airing cupboard near the hot water tank - remembering to check regularly for germination.