Durham University Botanic Gardens in June

Predictably, perhaps, June has been no better than May from the point of view of having time to wander in the Botanic gardens but I can’t pretend I’ve always had my nose to the grindstone marking!  June included a week of amazing weather exploring Arnside and Silverdale AONB with second year Durham Ecology students, where I learnt as much as them about moths, birds and small mammals, rather than focussing on the plants with which I am so familiar.  The end of June also brought a much-anticipated visit to the Knepp rewilding project, in west Sussex, where we saw how a low density of large herbivores such as Longhorn cattle, Tamwoth pigs, Exmoor ponies and deer can be used to drive habitat creation and produce functioning ‘natural’ ecosystems with minimal input from humans.  More on that in due course, I’m sure.

June is peak flowering time in the Botanic gardens and elsewhere for many of our most important pollinators, so it was good to see plenty of them enjoying the Geraniums and huge Echiums on the sunny morning I visited; a mixture of bees, wasps and flies which I am way too inexpert to identify. Geraniums rely heavily on nectar guides to attract the bees and hoverflies which are their main pollinators – these direct the insects to the centre of the flower where the pollen and nectar can be found.  As geraniums are native throughout most temperate and montane parts of the world, from the Mediterranean to the Himalayas, they clearly have a successful strategy for reproduction!  I suspect that the open flower form means a wide variety of insects can pollinate them successfully.

I have several attractive varieties in my garden and there are more in the Botanics but I would argue that the wild forms, both in the UK and elsewhere, are equally stunning.

Wild geraniums around the world. Clockwise from top left: Bloody cranesbill, Geranium sanguineum; Himalayan Geranium sp.; Wood cranesbill, G. sylvaticum; Herb Robert, G. robertianum; Meadow Cranesbill, G. pratense and G. wallichianum (Himalayas).

Echiums are in the Borage family, along with Forget-me-nots, Green Alkanet and Comfrey and also have a world-wide distribution. Depending on what you read, they get their name (which comes from the Greek echis, for viper) either from the fact that the seeds are the shape of a viper’s head or because the flowers have a forked stigma, which looks like a snake’s tongue.

Top: Echium vulgare (Viper’s bugloss) from Raisby Hill, Co. Durham and Bottom: Echium angustifolium, Cyprus

I’m guessing the gigantic ones in the Botanic gardens, considerably taller than me, are E. pininana or Tree Echium.

Echium pininana in the University Botanical Gardens

Echiums and other Boraginaceae rely on the production of abundant nectar to attract their pollinators rather than visual cues – you can see at least five on the two spikes in the photo above.  The efficiency of this pollination and seed set makes many of the Boraginaceae the bane of gardeners’ lives – I know to my own cost that starting with one Borage plant soon leads to multitudes!

The third group of plants which were particularly conspicuous during my visit in early June were the ‘Candelabra’ primulas in the woodland garden.  This group of primulas come mainly from the far east and have whorls of flowers in jewel-like colours spaced out up the stem.  They tolerate heavy, damp soils and I’ve long been coveting some for the garden so the new pond was the perfect excuse to buy a lovely red one from my local nursery!

Like other primulas, the candelabra species have flowers of two forms – either pin (long style carrying the female stigma and short filaments carrying the anthers) or thrum (short style and long anther filaments) – a phenomenon known as heterostyly.

Drawings from Darwin’s book of pin-headed (left) and thrum-eyed (right) Primula flowers

According to Phil Gilmartin, who works on the genetic basis of heterostyly, Darwin was not the first to notice that Primula flowers came in two forms but he was the first to work out at least part of the reason for it. On a purely physical level, it is easier for an insect to transfer pollen between different flowers with reproductive structures arranged like this and so genetic variability is promoted. But Darwin was also able to show that more seeds were set when a pin and thrum Primula were crossed than two flowers of the same form and that the progeny of such a cross would always be 50 % pin and 50 % thrum in form. We now know that there is also a level of genetic incompatibility at play, involving a self incompatibility (SI) gene, with two alleles. Thrum plants are heterozygous for this gene, carrying one dominant and one recessive allele (genotype Ss), whilst pin plants have two recessive alleles (ss).  There is still plenty of debate about the evolutionary relationship between heterostyly and self-incompatibility – which came first, or did they evolve together? Did heterostyly evolve to promote insect-mediated out-crossing as Darwin asserted or does it also have an important role in restricting self-pollination (Gilmartin, 2015)? Gilmartin’s paper makes an interesting read, tracing the first observation of heterostyly in Primulas back well beyond Darwin to the writings of Clusius in the late sixteenth Century. Darwin’s unique contribution was in beginning to understand its significance.

Gilmartin, P. (2015) On the origins of observations of heterostyly in Primula. New Phytologist, 208, 39-51 (available at: On the origins of observations of heterostyly in Primula (wiley.com)

Darwin C.R. (1862) On the Two Forms, or Dimorphic Condition in the Species of Primula, and on their remarkable Sexual Relations.  Journal of the Proceedings of the Linnean Society of London (Botany) 6: 77-96. (Available from darwin-online.org.uk)


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