Tuesday, April 23, 2013

A Conference, Darwin and plant sex

April 5-7 of this month was the first conference of the Master Gardeners of Ontario. It was held at the Deerhurst Resort just outside of Huntsville on a cold and snowy weekend. We had master gardeners come to the conference from across Canada and one brave soul from Minnesota USA.

The conference was magnificently put together by many, many, hard working volunteers. It was an action- packed weekend with talks by, dare I say famous, Canadians in the gardening world, fascinating presentations by scientists, presentations by MG groups that highlighted their achievements, games, contests, a dress- up evening, and lots of great food and drink, all with the great Canadian outdoors nearby.

Of the three keynote speakers we heard, I think my favourite was the presentation by Dr. Spencer C.H. Barrett of University of Toronto, called 'Darwin's botanical legacy and its relevance to the appreciation of the modern garden' , a mouthful to be sure. He spoke about Darwin's work on botany and mentioned that despite having written six major books on botany this is the area where he is least known.

From the introduction:
It is not well known that Charles Darwin worked on plants more than any other group of organisms. For over 40 years he made observations and conducted experiments on plants at Down House in Kent, England. He wrote six major books on botany, three of which focused on plant reproduction. He also wrote many practical articles in Gardener' s Chronicle and maintained a glasshouse of diverse plants. His botanical works have provided the conceptual foundation for understanding floral adaptations that promote cross-fertilization and also the mechanisms responsible for evolutionary transitions in plant reproductive systems. Many of Darwin's insights remain remarkably durable today and have stimulated much current research. Here, I review Darwin' s seminal contributions to plant reproduction and provide an overview of the current status of research on several of the main  topics to which he devoted considerable effort. This will include the pollination and mating biology of plants, why some give up outbreeding altogether, and why others become invasive.

I thought I would try to pass on something of what he said regarding why some give up outbreeding altogether, and why others become invasive. If I get it wrong, the mistake is mine entirely.

94% of plants are hermaphroditic, that is, having both male and female organs. Hermaphroditic plants can self pollinate. With such an enormous percentage of plants that can self pollinate Darwin raised the question, why are there not more of all these plants, in effect, why are we not overrun with these plants? Plants and animals  that inbreed increase the chance of offspring being affected by recessive or deleterious traits, so how is it that this does not happen in the plant world to the degree one would expect?

Darwin's interest in these questions was not purely scientific. He had fallen in love and married Emma Wedgwood, his first cousin, and they were eventually the parents of 10 children, three of whom died at early ages. Marrying a cousin in this time was probably a little scandalous, but more disturbing for Darwin was the understanding that genetically he and his wife were very similar and the creation of children may lead to deleterious traits in their offspring.
Let us return to the plants he studied.

So how is it that we are not overrun with pansies? Well, hang on to your hats, here is some science. Hermaphroditic plants (and that is most of them) fall into two or three morphologically distinct mating groups where the sex organs, (stamen and pistil) differ in length. This is called heterostyly and occurs in plants that have more than one form or morph (like the trillium of which there is the red morph and the white morph).  The flower may be distylous with two flower morphs, where the stamens are short and the pistils are long, or the stamens are long and the pistils are short, or they are of the same length. Or the plants may have three flower morphs are termed tristylous. Each morph has two types of stamens. In one morph, the pistil is short, and the stamens are long and intermediate; in the second morph, the pistil is intermediate, and the stamens are short and long; in the third morph, the pistil is long, and the stamens are short and intermediate.
Within each flower morph the sex organ has different sizes meaning they are not compatible sexually and therefore can not self pollinate. So in layman's terms, because the pistil and stamen are at different lengths the plant is unable to self pollinate.

photo source

Another example of plants that prevent self pollination are flowering  plants that are entirely male, or entirely female. Most Hollies (Ilex ) are either male or female plants and need to be grown within a close proximity to each other in order to ensure cross fertilization. Yet another example, in some plants, the pistils and stamens of the same flower will mature at different times, again to prevent self pollination. For example, the pollen may mature before the stigma is chemically receptive to being pollinated. In other plants, the stigma can chemically detect whether or not the pollen has come from the same, or a different plant, and prevents self pollination from occurring.

So it turns out that nature has figured out ways to prevent one plant population from overcrowding others thereby reducing the recessive and deleterious affects on the gene pool. Instead diversity is created and the improved fitness of the species.


  1. Nature is so wise. :o) It's interesting that nature has solved the problem of finding a suitable partner by making most plants hermaphroditic. If more invasives were able to self pollinate, we'd be done for. Excellent info!

    1. Thanks. I really enjoyed his talk and thought the information interesting.

  2. Your conference sounds great. You did a great job of explaining all that pollination stuff which I know but like to brush up on from time to time.

    1. Pollination is a sticky business and leaves me confused quite often. I am not sure how long I will remember what I learned.

  3. Bardzo ciekawa konferencja i poruszone tematy. Pozdrawiam.
    A very interesting conference and the issues raised. Yours.

  4. Really interesting Patty. I just did a post today on invasive plants that do command entire habitats and they do it by both underground rhizomes and seed. Nature gives a little extra to some plants over others though. I did learn in MG classes about plants not self pollinating by having differing sizes sex organs. I wonder who was our speaker then? I would have liked going to the conference. I always forget about putting it on the calender.

  5. I will remind you of the conference next year, or whenever they hold the next one. There were some very interesting topics.