Various Contrivances

3D front cover of The Formation of Vegetable Mould through the Action of Worms by Charles DarwinMassively exciting news from the Cambridge University Press conservation area this week! I was chatting to a colleague in our ‘orchard’ when I suddenly noticed a bee orchid flowering its head off in the long grass a few feet away. The colleague, a technical wizard, kindly took some photographs with his phone, and here is the plant:

Bee orchids

Later, a more knowledgeable other colleague spotted another plant at the other end of the orchard – so we may be starting a colony. How did the seeds get into our space? I knew that they are as fine as dust, and are distributed by wind, but apparently they rely on raindrops to bring them down to earth – what a wonderful image! – and are then very fussy about the sort of soil they land in, so we feel very privileged that they approve of our sandy, gravelly and rock-hard terrain.

Another thing I thought I knew about bee orchids (Ophrys apifera, given its Linnean name by William Hudson, the praefectus horti at the Chelsea Physic Garden) was that the flower had evolved into its characteristic form to attract bees, vital to pollination, who would attempt to mate with it, and cover themselves with pollen in the process.

One of the many joys of working on the CLC is that we don’t have to step far to find a copy of a helpful book – in this case, Charles Darwin’s On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects.  He has a whole section devoted to the bee orchid, but to my surprise, he is quite clear that, in Britain at any rate, the plants are self-fertile, requiring the slightest of air movement for the pollen to fall on the stigmas. He attributes this discovery to Robert Brown, but proceeded to examine, observe and ponder all the bee orchids he could lay his hands on in order to validate or confute the theory.

(Robert Brown, by the way, is the same man who was to have written Sir Joseph Banks’s biography. He had sailed with Matthew Flinders on the Investigator voyage; he first described the physical phenomenon which is named ‘Brownian motion’; he wrote papers on the botanical discoveries made by Arctic expeditions such as those of Ross, Scoresby, Franklin, and Parry,  and – in an extraordinary coincidence  – it was at the extraordinary meeting of the Linnean Society to mark his death in 1858 that Darwin’s and Wallace’s explosive papers on natural selection were read out to a not altogether receptive audience, the rest REALLY (for once) being history.)

Reading the nine pages which Darwin devotes to the fertilization of the bee orchid brought home to me the staggering patience of the man. Famously, he might never have published On the Origin of Species, despite nagging from Huxley and Hooker, if he had not been spurred on by Wallace’s parallel discoveries, but the upside of this apparent procrastination must have been the thoroughness of his approach and the exacting detail of his observation.

First, there is the minute description of the structure of the plant, and the way in which pollination takes place. Then there is the exclusion of other possibilities: ‘To make sure that no other aid [than air movement] was requisite, though the experiment was superfluous, I covered up a plant under a net, so that some wind, but no insects, could pass in, and in a few days the pollinia had become attached to the stigmas; but the pollinia of a spike kept in water in a still room, remained free, suspended in front of the stigma.’

Next Darwin considers the way in which the form of the flowers follows the function:  ‘it is impossible to doubt that these points of structure and function, which occur in no other British Orchid, are specially adapted for self-fertilisation’. Moreover, by careful observation of bee orchids near Torquay, Darwin produced the statistic that almost every flower resulted in a seed capsule, whereas fly orchids, which do need insect pollination, produced only seven seed capsules from forty-nine flowers. Moreover, in north Kent, in 1860, he examined ‘twelve spikes bearing thirty-nine flowers, and three of these had one pollinium removed, all the other pollinia being glued to their own stigmas. In another lot, from another locality, however, I found the unparalleled case of two flowers with both pollinia removed, and two others with one removed. I have examined some flowers from south Kent with the same results…’. A fellow naturalist, A.G. More, on the Isle of Wight, surveyed 136 flowers for Darwin, which the same conclusions, and ‘I will only add that I have never seen an insect visit these flowers.’

Q.E.D., you would think. But being Darwin, he needs to look at the other side of the case. The bee orchid, in terms of its structure, appears to be ideally suited to be pollinated by an insect sticking its head into the flower and having the sticky pollinium attach itself – which is more or less what happens with the fly orchid (Ophrys insectifera) and the early spider orchid (Ophrys spegodes), for both of which the insect is vital. The form of the flower did not evolve by chance – therefore there must be occasions on which a bee does in fact fertilise the plant, and thus assure the cross-fertilisation which would, in the evolutionary time-scale, prevent the species failing because of in-breeding. ‘Thus the generative functions of this plant would be brought into harmony with those of other Orchidaceae, and, indeed, with those of all other plants, as far as I have been able to make out their structure.’

In fact, according to Kew, Mediterranean types of bee orchid are indeed pollinated by insects: ‘In other Ophrys species in the Mediterranean region, for example, male bees or wasps try to copulate with the lip of the flowers, which look and smell like the females of their own species.’ The smelling mimicry is an especially subtle evolutionary touch. So Darwin was right and right – as in the similar case when he hypothesised the existence of a moth with a particularly long proboscis which could fertilise the Angraecum sesquipedale of Madagascar. Darwin had been given some of the flowers by James Bateman  (the wealthy orchid enthusiast who later laid out the fantastical gardens of Biddulph Grange), and he experimented to find ways in which the pollinia could be extracted from the flower. He concluded that it would require a moth with a proboscis ten or eleven inches long.

After the publication of Darwin’s book, the 8th Duke of Argyll went into print with The Reign of Law (1867), which specifically criticized this theory and argued that a plant species of such complexity was evidence of Creation, not Evolution. Alfred Russel Wallace then stepped into the fray with a paper called ‘Creation by Law’, which demonstrated that the co-evolution of the plant with such a moth could arise through natural selection. And in 1903, the moth was discovered, and named Xanthopan morganii praedicta (the latter precisely because Darwin had predicted it).

Darwin’s discussion of Angraecum sesquipedale is prefaced by an apology: ‘I feel the reader will be wearied, but I must say a few words…’. One imagines him, year after year, stooped with a magnifying lens over growing plants, or flower stems in water, or herbarium species, teasing out structures, making comparisons, examining more and more specimens before committing himself to publication. And consider the forty-year outdoor experiment at Down House to observe the effect of earthworms on soil. Millions of people will have noticed that worms pull surface vegetable debris into the soil: only one man had the interest and the patience to observe over decades exactly what they did, and what effect this microscopic activity had, and continues to have, on the planet.

Caroline

This entry was posted in Biography, Gardening, Life Science and tagged , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s