The Monthly Orchid—a Letter to the Earth

… its broad extremity is folded over and backwards in a peculiar manner, so as to form a sort of shoe ... Hence arises the English name of Lady's Slipper. (photo by Matt Lavin).

Let me start by explaining how honored I was when Ms. Plantsandrocks asked me to contribute to her series about Orchids while she is traveling in The Dakotas. My knowledge of the family is vast, but I fear it may be a bit outdated. However, the heart-felt joy that I find in Orchids has not diminished, and I hope to share some portion of it with you. By the way, I was advised to write in a straightforward manner, as the eloquence of my day has gone out of fashion. I shall try my best in that regard, but can make no promises, being very much a man of my times. (1)

I believe some of you are acquainted with my book, Fertilisation of Orchids, described by Ms. Plantsandrocks in an earlier article. It was issued shortly after On the Origin of Species, with the intention of demonstrating that I had put much effort into finding evidence and case studies in support of natural selection. During the summer of 1839, and the one previous, I had been strongly drawn to the cross-fertilisation of flowers by insects, having come to the conclusion that crossing played an important part in keeping specific forms constant. I attended to the subject during every subsequent summer, my interest greatly enhanced by having read C. K. Sprengel's wonderful book, Das entdeckte Geheimnis der Natur.

The Secret of Nature in the Form and Fertilisation of Flowers Discovered, CK Sprengel 1793 (source).

My little orchid book cost me ten months’ work, but it was a sound investment. Since its appearance, a surprising number of papers and separate works on the fertilisation of all kinds of flowers have appeared; and these are far better done than I could possibly have effected. The merits of poor old Sprengel, so long overlooked, are now fully recognised many years after his death.

Let me move now to the Orchids, specifically the Lady’s Slippers, which I was told grow in The Dakotas in the heartland of America (2). However, in case any one should visit this post who has never attended to Botany, it may be convenient to explain the meaning of the terms used. We shall look at flower parts for which Orchids are distinctive, in comparison with common flowers. It may be helpful to follow the illustration immediately below (not mine; provided by Ms. Plantsandrocks).

A typical orchid flower (Sereno Aceto).

In common flowers, the stamens (male organs) surround one or more female organs, called pistils. In contrast, almost all Orchids have only one stamen, which is confluent with the pistil forming what we call the Column. As in common flowers, the Orchid stamen has an anther which carries pollen—the male vivifying element. However, in contrast with the pollen of common flowers, which consists of fine granular powder, in most Orchids the grains cohere in waxy masses called Pollinia.

Regarding the female organs, most Orchids have three united pistils. The upper part of the pistil is soft and viscid; this is the stigma. During the act of fertilisation the stigma is penetrated by long tubes emitted by the pollen grains; these carry the contents to the ovules (young seeds). However, the stigma of the uppermost of the three pistils has been modified into an extraordinary organ, called the Rostellum to which the Pollinia are attached! The Pollinia along with part of the Rostellum are removed by insects, and perhaps transported to another plant of the same species.

I will describe only briefly the outer divisions of the Orchid flower. The three outermost are the Sepals, generally coloured like the three inner ones, the Petals. The lowest Petal is larger, and often assumes a most singular shape; it is called the lower lip, or Labellum. It secretes nectar, thereby attracting insects.

The Labellum forms the slipper of the Greater Yellow Lady’s Slipper (MWI).

Now we shall discuss the genus Cypripedium, the principle object of this article. It is quite unusual, differing from other Orchids far more than any other two do from each other. An enormous amount of extinction must have swept away a multitude of intermediate forms, and left this single genus as a record of a former and more simple state of the great Orchidean Order. So let us look closely at the Lady’s Slipper, perhaps one of the most primitive of today’s Orchids.

Cypripedium pubescens; Catesby 1754, The natural history of Carolina, Florida ... (source).

Species in the genus Cypripedium possess no rostellum, and all three stigmas are fully developed and fully united (s. in illustration below). The single anther characteristic of all other Orchids is here rudimentary (a.), while the two anthers reduced in other Orchids are here fertile (a’.). Finally, the pollen-grains are glutinous and not united into waxy masses. There are other fascinating differences, but these are too minute and complex for anyone who has not a strong taste for Natural History. If you would like to know more, please refer to my book, starting on page 270.

Cypripedium, upper part of Labellum removed. I am much indebted to Mr. G.B. Sowerby for the pains which he has taken in making the Diagrams in my book as intelligible as possible.

We arrive at last at fertilisation and the role of insects, a subject to which I have devoted much time and thought. But first I shall explain the context in which I struggled—likely it will strike you modern thinkers as preposterous. In my time it was widely assumed that most flowers, being hermaphroditic (having both male and female parts), were self-fertilising. Even more shocking, it was only during the early years of my century that the idea of sex in plants finally emerged from the mists of profitless discussion and feeble experiment (3).

From Die Alpenpflanzen nach der Natur gemalt, F. Tempsky,1879-1884 (source).

At first glance the Lady Slipper appears to present an obstacle to fertilization by insects. As you can see in the illustration above, its broad Labellum is folded so as to form a sort of shoe, the end closed to any entry. However, close examination reveals two narrow slits through which an insect can reach with its proboscis the nectar waiting at the bottom of the Labellum. Most importantly, it can only do this by brushing against the two fertile anthers, thereby smearing its proboscis with the glutinous pollen. 

I tested this myself, by inserting a bristle which became smeared with pollen. We thus see how important, or rather how necessary for the fertilisation of the plant, is the curious slipper-like shape of the Labellum, in leading insects to insert their probosces by the lateral passages close to the anthers. The efficacy shown by nature is truly striking.

To those of you who have stayed with me to the end of this article, I am immensely grateful. More than a century has passed since I last spoke of Orchids and fertilisation with fellow naturalists on Earth, or so I was told (I have no sense of time passing). It is my sincere hope that you encounter this beautiful flower in its natural setting, and that you look closely and take the time to appreciate the cleverness of Evolution’s hand (4).

Charles Darwin, M.A., F.R.S., &c.

Greater Yellow Lady's Slipper in the wilds of Minnesota (MWI).

Notes

(1) This article was edited for greater concision and clarity, but without altering style or content. However, do NOT use it to quote Mr. Darwin. Instead use the Sources listed below.

(2) The yellow-flowered Lady’s Slipper in South Dakota is Cypripedium parviflorum var. pubescens. Its synonymy is lengthy: C. pubescens, C. flavescens, C. calceolus, C. calceolus var. pubescens, C. parviflorum var. planipetalum, and C. veganum. Our white-flowered species are C. candidum and C. montanum.

(3) This was explained by the author’s son, Francis, in his introduction to Chapter VII, "Fertilisation of Flowers" in The Life and Letters of Charles Darwin. See page 257.

(4) In South Dakota, Cypripedium parviflorum var. pubescens, the Greater Yellow Lady’s Slipper, has been found in wet meadows in the far eastern part of the state.

Sources

Barlow, Nora (Darwin's granddaughter), editor. 1958. The Autobiography of Charles Darwin, the first complete version. Darwin online

Darwin, Charles. 1862. Fertilisation of Orchids. On The Various Contrivances By Which British And Foreign Orchids Are Fertilised By Insects, And On The Good Effects Of Intercrossing. Darwin Online

Darwin, Francis, ed. 1887. The Life and Letters of Charles Darwin, including an autobiographical chapter. Volume 3. Darwin Online