Field of Science

Millions of little plant mouths

Now for something a bit different.

You may have heard of stomatal guard cells back in highschool. Something about shrivelling up to close when hot, potassium ion channels and carbon dioxide? Yeah, for most people they fade forever into the vague memories of cramming for a science test. The only lasting effect of this exercise tends to be an aversion to anything botany.

That's how it worked for me anyway. I recall finding plants to be the most boring unit in highschool biology, despite having a good teacher. They fail to convey the wonders of land plants, instead falling back on dry diagrams and that wretched photosynthesis 'equation'.

Of course, one can't really blame them -- the teachers themselves have learned it the same way. And you simply cannot even begin to share the amazing world of stomatal development in a highschool classroom. After all, I've worked hard over a year in the field, and I'm still just a novice. But I do believe the approach could be altered to make the time used in a more constructive, and enjoyable, manner for both students and teachers.

I think part of highschool education should promote asking good questions. There is almost no aid in that aspect of learning -- information is just presented, and some students never realise the diversity of questions they could ask about it.

First of all, why do plants even bother? A stoma is a fairly complex structure:

(Fred Sack)

Bilateral symmetry requires a coordinated process of development; special care must be taken to ensure spacing between stomata; the guard cells must maintain a kidney shape for optimal functioning. Such structures must be really worth the trouble to evolve!

Plants, like most terrestrial life, have their origins in water. Their ancestors looked something like this:


This Charophyte lacks stomata. Living in water, your cells are already in the aquatic environment they are specialised for. There are some special adaptations for dealing with freshwater due to differences in osmotic pressure, but we'll overlook those.

The problem with migrating onto land is that cells require an aquatic environment to function. When our ancestors left water, they had to figure out a way to carry the aquatic environment with them. Fungi generally live inside their food, or in soil; still, they have a chitinous wall for protection and structural support. Our lineage came up with skins and an intricate system of vessels.

Some plants do quite fine sticking to damp areas and not dying upon dessication. Surviving drying up was probably already advantagous enough for aquatic plants, since ponds are prone to drought. Liverworts are only a few cell layers thick, lack a vascular system, and have a very thin coat of wax on the surface:

(I think I'll just have to introduce thallose liverworts someday -- they're awesome!)
A good bryophyte reference here

Being thin enough and living in damp places allows you to skip stomatal development:


However, as plants move into dryer places and gained size, dessication becomes a problem. The solution? A think layer of cuticular wax to stop water evaporation:

(Lacey Samuels)

Now we have a problem. Wax doesn't breathe very well. You need to inhale CO2 for photosynthesis; you need to exhale O2. Having open holes in the waxy cuticle defeats the very purpose of having one, as well as exposing the organism to invaders. That's where stomata come in. Stoma is Greek for 'mouth', and that's pretty much exactly what these structures are: millions of mouths covering the leaf surface, enabling the plant to breath while regulating water loss.

There are also specialised uses for stomata as a result of structural neofunctionalisation, such as nectar secretion. Some plants even have stomata on their roots -- apparently the mycorrhizal associations (fungal-plant symbiosis) began by hyphae penetrating open root stomata!

Early stomata consisted of a single cell with an opening in the middle, much like this fern stoma:

(source PJ Franks, Plant Physiol)

In flowering plants, stomata contain two cells instead, perhaps resulting from an extra (symmetrical) division somewhere in the evolutionary past. Aren't they cute?

Next question: How do they 'happen'?
Next time~

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