The past couple of posts have had to do with plant movements.
And vining plants move up vertical supports so as not to lose their photosynthetic height advantage over their more recumbent competitors for sunlight.
The quickest, most dramatic, and most unusual example of plant movement, however, is probably the Venus flytrap. Its specialized leaves must move very fast or lose its insect prey.
Don’t Move A Muscle
Aside from very unusual plants such as the Venus flytrap and the “touch-sensitive” plant Mimosa pudica, we don’t notice most plant movements because plants move outside our normal “time frame”. (As previously posted, “plant time” is typically thousands of times slower than “people time”.)
But even though we don’t normally see it happening, all plants can move. (This fact is well-described in the books The Power of Movement in Plants and The Restless Plant.) It’s also well-known that plants don’t have muscle cells, like animals do. So, how can plants move without muscles?
In general, they do it two ways – sometimes hydraulically, but mostly developmentally.
Plant Cells Under Pressure (Or Not)
Most relatively rapid plant movements (think Venus flytrap or Mimosa pudica or leaf sun-tracking) happen over the course of seconds to minutes. Such movements are usually hydraulic in nature. That is, movement is achieved through changes – sometimes very rapid (seconds) – in the hydraulic pressure of plant cells.
(Although I’ve briefly described this in a previous post on hydraulic spring mechanism in the Venus flytrap, let’s go into a bit more of the story here.)
Unlike typical animal cells, plant cells are usually pressurized. That is, they have a hydraulic pressure (a.k.a., turgor pressure.) They can do this because they are usually surrounded by a rigid cell wall, unlike most animal cells that don’t have cell walls. (And this is why animal cells don’t have hydraulic pressure – because they’d burst!)
A good analogy is a bike tire. Think of the cell as the inner tube. Think of the cell wall as the tire. If you try to blow up the inner tube without the tire surrounding it, the inner tube will eventually swell up and rupture. When the inner tube is enclosed in the tire, the inner tube can then be pressurized.
Perhaps a better analogy is to use a water balloon and a shoebox. Fill up a water balloon with the garden hose, and it’ll eventually burst. This is the animal cell. Now put the water balloon inside of a sturdy shoebox and put on the lid. Filling the water balloon inside the shoebox with the garden hose will cause the water balloon to push up against the shoebox. The water pressure within the balloon will increase to the same pressure that is in the hose. The water ballon will be under pressure, but the water balloon won’t burst.
Since plant cells are enclosed within the plant cell walls, they can become pressurized.
How do they get pressurized?
Do plant cells have water pumps on their surface to pump water in and out? No.
Water flows in and out of plant cells osmotically. That is, to accumulate water and become pressurized plant cells must first accumulate osmotic solutes such as salts and sugars. The higher the solute concentration inside the cell, the higher the attractive force for water becomes. As water flows into the plant cell the turgor pressure increases. The turgor pressure will eventually equal the attractive force for water, which is referred to as “equilibrium”.
Anyway, back to hydraulic plant movement.
At the base of plant leaves that move hydraulically one can usually find something called a pulvinus. This is a group of cells that hydraulically can move the leaf. Think of the leaf like a hand. The pulvinus would be analogous to the wrist.
When all the cells in the pulvinus are turgid, that is, fully pressurized, the leaves are open in Mimosa pudica and the Venus flytrap, for example. What happens in these plants is that touch elicits an electrical signal, which triggers the opening of salt “gates” in the pulvinus cells. The salts (primarily potassium ions) flow out of the cells, and the water follows. Thus the cells become depressurized in seconds. In Mimosa pudica this causes the leaves to quickly fold in. And in the Venus flytrap, it causes the leaves to close on the prey.
This is basically the same mechanism that is responsible for so-called nyctinasty (a.k.a., diurnal “sleep movements”) of leaves (think bean or clover plants) or a flower petals (think tulips or poppies) that sometimes occur in the late afternoon.
A related but much more complex phenomenon is heliotropism, that is, leaf sun-tracking. In this case, the leaf blades, and sometimes flowers, track the sun during the day such that the leaves remain perpendicular to the sun’s rays. These represent more examples of hydraulically controlled movements in plants. (More on this later on.).
Hydraulic plant movement is pretty cool, but most cases of plant movement turn out to involve growth, not hydraulics. Hydraulic plant movements can be reversible, but growth movements or “tropisms” are irreversible.
Phototropism, gravitropism, and thigmotropism (vining and tendril curling) are all common examples of “differential growth” responses. That is, the bending of the shoot or the root or the curling of the vine or tendril is due to more growth on one side of the root, shoot, or tendril compared to the other.
How this happens is that, collectively, the cells simply grow longer on one side than the other. The only catch is that this can only happen in growing parts of the plant.
And since this differential growth takes hours or days to occur, this is why these plant movements are so slow.
So if you’re growing in order to move, then you truly are living on “plant time”.
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