Trop Picks: Darwin’s orchids

For Charles Darwin, orchids were no less central to his theories of natural selection than apes are to our understanding of human evolution.

In a typical flower, the male and female reproductive parts--i.e., stamens, topped by anthers containing pollen, and carpels, topped by a stigma that receives the pollen--are separate.  

In orchids, the stamen and stigma are fused into a single column, with the stamen holding the pollen in a large sticky clump just above the stigma.

Given that the pollen and stigma are so close to each other, you’d think that the easiest way for the orchid to reproduce would be to self-fertilize.

In On the Origin of Species, Darwin argued that where possible, organisms avoid self-fertilization in favour of cross-fertilization, in order to make their offspring more robust.

He noticed that, between the pollen and stigma of the orchid, there was a protuberance, known as the rostellum, that blocked the path between the two.

Darwin hypothesized that the purpose of the rostellum was to prevent self-fertilization in the orchid in all but the most desperate circumstances. 

But even if cross-pollination via pollinators was the preferred method of pollination, there was still the issue of how the pollen from one orchid was able to be transferred to the stigma of another. 

Once the sticky pollen had made its way on to the back of the pollinator, the angles of all the players in this reproductive dance appeared to facilitate the pollen sticking to the anthers of the second plant and not its stigma. Therefore, some sort of adaptation had to occur between the time the insect left the first plant and entered the second. 

To prove his hypothesis, Darwin inserted a sharpened pencil point into the first orchid to extract its pollen. As he moved the pencil from the first orchid to the second, he noticed the stalks holding the pollen drying out, and as they did so, the position of the pollen altered from a vertical to a horizontal one, the right position from which to land on the stigma.

Of course, the timing of the process had to be extremely precise, so that the pollen would assume the position exactly as the pollinator entered the second orchid.

The amazing contrivance by which this process occurs, along with other such contrivances, were all outlined in Darwin’s book, The Fertilization of Orchids: On the Various Contrivances by which British and Foreign Orchids are Fertilized by Insects, and on the Good Effects of Intercrossing. 

Orchids are an exceptionally diverse family of around 26,000 species of flowers. These 26,000 species are all outcomes of a single ancestral flower’s struggle to propagate itself. Since Darwin’s day, advances in technology have allowed botanists to discover even more examples of natural selection within orchids, such as food deception, sex deception, variations in colour and shape and so on.

Interestingly, Darwin’s use of the term “contrivances” through the book led many to think, mistakenly, that he was admitting to an intelligent creation. His friend Asa Gray, a botanist at Harvard, teased him saying that “if the Orchid-book…had appeared before the Origin, the author would have been canonized rather than anathematized by the natural theologians” (as quoted in Boulter).

In fact, the opposite was true: Darwin thought the “amount of modification, cohesion, abortion and change of function” on display in the orchid world pointed to a messiness and eccentricity that one would not expect of a divine mind.

Sources:

Jim Endersby in Orchid: A Cultural History, 2016.

Peter Bernhardt and Retha Edens Meier in Darwin’s Orchids, 2014.

As quoted in Michael Boulter’s Darwin’s Garden: the Origin of the Species, 2008.