There’s a lot less sperm around than most people realise

Pollen is not a gamete. It's a spore.

Misconceptions about plants have more lives, and much longer lives, than a cat.

 

Man sneezing in a canola/oil seed rape field.
A misunderstanding gets up his nose. Photo: Bigstockphoto

As AoB Blog’s non-botanist, I’m used to there being plenty of things I don’t know about plants. I’m usually good for a “Wow!” if anyone mentions that a line of plants has been around since the age of the dinosaurs, regardless of how many times I’m told this about plants. But there’s another problem. Sometimes I can learn a fact and be completely unhindered by any understanding of it.

An example is the post from the Phytophactor Pollen is not plant sperm. I should know this. I read about the alternation of generations when reading up about mosses, and pollen is a perfect example of this. I know some people reading will know almost as little about plants as me, so I’ll try to explain this below (or just demonstrate my misunderstanding) while botanists can skip to the next bit where I ask why should this be news?

Plant reproduction is weird

…or at least weird by human terms. Humans are dipolid. We have pairs of chromosomes. We produce a gamete which is haploid. These are cells that carry single chromosomes. They combine and make a new diploid human. So one diploid human produces another diploid human. Plants do not do this.

We’ll start by looking at angiosperms, flowering plants. Let’s imagine an oak tree. An oak tree is diploid, if we ignore the ones that aren’t. If pollen is sperm then we’d expect them to produce lots of the stuff and for some of it to fall on eggs to fertilise them. But when you look closely that’s not what happens.

Pollen is not a gamete.

It’s a spore. It grows into haploid pollen grain that can produce gametes, but it’s not just the gamete. The pollen travels to a pistile and develops a pollen tube to deliver the gametes. This can be quite complex. The seed is produced and this becomes a diploid plant that starts over again. Something like a tree that produces pollen and ovules is called a sporophyte because it produces spores as pollen or ovules. The pollen or ovule is called a gametophyte because they produce gametes.

So it’s complex, but is it so bad to think of pollen as sperm? It’s effectively doing the same job. Maybe, but things like mosses complicate things further.

Mosses alternate generations too, but the mosses you see are the gametophyte stage. The sporophyte stage, the equivalent of a tree, is tiny and only lives off the gametophyte. As the Phytophactor points out, it’s the same process but the other way round.

Why does the idea of pollen as sperm persist?

“TPP wonders how many times he’s explained this during his career and seemingly with no impact what so ever except perhaps on a case by case basis.” says the Phytophactor.

My first thought was that we don’t experience the word on a microscopic basis. It sounds like a good reason, but it can’t be right. We take germs seriously, but they’re microscopic. So the hidden nature of pollen and ovules isn’t enough to explain why we get them so badly wrong. I think it’s also that our senses tell us a different story. Seeds are produced on the tree and these become plants. It’s not simply that we don’t see the alternation of generations, it’s also that we see apparently contradictory evidence with our own eyes.

The evidence is only apparently contradictory and it would be clear that something different is happening if we looked closely, but why would we? Our thoughts about reproduction are dominated by how our own bodies work and what we erroneously think about plants is consistent with that. Even with weird sex, when Captain Kirk decides it’s time to kiss the alien’s daughter he might notice how green she is, or be surprised by an extra tentacle, but he never discovers she’s microscopic and one of thousands.

The token romance element of sci-fi films could be a lot more complex.

…and maybe that’s the biggest barrier of all. We like to see ourselves as the height of complexity. Really grasping what the alternation of generations means doesn’t just run counter to experience. It also challenges our chauvinism. Yet plant reproduction is not passive or simple. Clearly people can understand it, but it’s not an easy tale to sell. It’s also something that plants are very good at handling for themselves. Most people don’t intervene in their plants’ sex lives they way they would for a dog or a cat so it’s not a tale we necessarily need to know.

If leaving your gametophytes un-neutered meant you woke up one morning and your kitchen was a small forest that needed toilet training then I bet people would become staggeringly well-informed about plant reproduction. That sounds like a challenge for the GM people to sort out.

  • Do I get to put Alun in my “win” column? Sounds like you got it better than my plant diversity students. So next: flowering plants, where an ovary is not a sex organ and an ovule is not an egg. Enjoy!

  • You can put me in the win column, but it’s so bizarre that I’m bound to get carried away with it and start saying things like: “Wow! The average oak is microscopic and any trees you do see can be ignored as statistical flukes!” 🙂

  • Yes, Alun goes in the win column! A possible partial explanation for the widespread belief that pollen is sperm may be that we botanists perpetuate it ourselves when we use the “ick” factor to grab students’ attention. Once the image of “sperm up your nose” is in someone’s head, then it takes a LOT of “well, not exactly, let me explain….” to get that image out. Botany students will usually get it right eventually; but we really have to mind our language in guest lectures and intro courses. Thanks for the reminder.