learning in lectures is a two-way street

I really enjoy my first-year bio classes, & one of the reasons for this is that the students respond to my questions and ask questions of their own. I’ve just read Marcus’s excellent post on what he’s learned from his students & it’s spurred me to write a bit about this too.

So, what’s so good about student questions? Well, as Marcus says, those questions, & the students’ responses to our own probing, can combine to tell us (ie the teachers) a lot about our students’ current understanding of a particular topic. And if it turns out that they don’t follow it, or have particular misconceptions, then there’s not really any sense in going on to the next topic regardless. You’d just be muddying the waters further. Unfortunately there’s a tendency to push on anyway; after all there’s so much other material to get through & surely the students can read up about the bits they don’t get, after the lecture’s over? But it doesn’t work like that, so heaps of kudos to Marcus for throwing out 3 of the ‘set’ tut questions so that he & the class could focus on coming to terms with one key issue. (& if students in his class are reading this – you’ve got a really good teacher.)

The other thing is, you can just about guarantee that students’ questions will lead to me learning something new 🙂 Take moss, for example – a student question in a botany lecture led to my learning something quite fundamental about moss biology. So do bear with me while I set the scene…

You’ll sometimes see moss described as a ‘lower’ plant: mosses don’t have any xylem & phloem (the vascular tissues that transport water & nutrients around the plant. Their leaves generally lack a cuticle, which along with the lack of internal plumbing makes them very susceptible to dehydration; they don’t have roots, just ‘rhizoids’; and they use spores for dispersal.

By the way, mosses can tolerate extended periods of dehydration just fine. They go brown, shrivel up, to all intents & purposes look dead – but rehydrate them & bingo! they spring back into life. This poikilohydric lifestyle means that mosses can live in some pretty extreme environments, including mainland Antarctica (not the Antarctic peninsula), where they’re the most complex plant around. A bit like the plant equivalent of tardigrades, really 🙂

Anyway, back to the spores. Mosses, like all plants & in fact like algae as well, have a life cycle that’s characterised by something known as ‘alternation of generations’. In algae, mosses & ferns this manifests itself as 2 separate plants: a gametophyte generation, which produces the gametes, and the spore-producing sporophyte. (In the gymnosperms & angiosperms you never actually see a separate gametophyte, it’s tucked away inside the tissues of the sporophyte, which is the familiar pine tree or rose bush.) This is quite a complex way to do things, & among the bits which my students struggle with, & which we consequently spend a bit of time on, is the number of chromosomes in the gametophyte & sporophyte.

Because plants ‘do it’ differently from animals. In terms of chromosome number, gametes are haploid – they contain just a single copy of each chromosome. In animals, gamete production & the type of cell division that produces them, meiosis, are very closely linked. But that’s not the case in plants. Here, meiosis takes place in the sporophyte & produces, not gametes, but haploid spores, which are then shed & dispersed by the wind. When they germinate, they grow into the gametophytes, which are thus also haploid. Some gametophytes are female, & produce eggs; others are male & produce sperm (but by mitosis, not meiosis, so there’s no further change in chromosome number). When a sperm fertilises an egg, this produces a diploid zygote (ie two copies of each chromosome), which goes on to grow into the sporophyte. (Hopefully there’s an embedded video here, but if that doesn’t show for you, you’ll find it here on Youtube.)


Well, we’d spent quite a bit of one lecture going through this (& subsequently spent a fair bit of 2 more), & then someone said: but what determines whether a gametophyte plant is male or female? And do you know, I didn’t have a clue. It just wasn’t a question that I’d thought to ask myself, & maybe you can put that down to the fact that I’m really a zoologist by training rather than a botanist, & maybe I’d just never thought about it 🙂 But my goodness, once that student woke me up to the fact that here was something fairly central to the subject we were talking about, I went off & found the answer.

It turns out that in moss, all sporophytes are XY. This means that meiosis will produce 2 types of spore: half of them will carry a Y chromosome, & grow into male gametophytes. The other half have an X chromosome & become female gametophytes. Said like that, it seems quite straightforward, & I was mentally kicking myself for not having thought about that earlier. And when I went to the next lecture & shared what I’d found out, I also made a point of thanking the students for asking that key question in the first place. Because without that I might still be blissfully ignorant on that question (& yes, I’m sure there are many others!).

Learning in lectures does indeed go both ways.

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