Monday, May 01, 2006

Now Why Is That...?

Posted by Picasa

I'm
the
sensi
tive
type














This is puffball, one of the sensitive briars...not sure which one exactly and not anguishing about it. This one grows along the ground, amongst the grass in thick carpets of green and pink. The point of this post is the behavior of this plant and not it's fancy shmancy taxonomic name.

If you touch the leaves of this plant, they fold up immediately.
We could spend time wondering about how they know they've been touched, since plants don't have nerves...but that kind of thinking makes my head hurt...it's too soon after Grad Bash.

I prefer to wonder about the reason for this behavior. I am fond of telling my charges that every adaptation, whether structural or behavioral is a survival tactic. In effect, behaviors and physical structures aren't "just there", rather, they are only there because they offer improved chances for survival.

So I wonder about this behavioral adaptation of the puffball. If I squint, I can sort of picture the mechanics involved...it seems reasonable that a sudden drop in turgor pressure could cause a quick wilt. I picture the plant emptying water vacuoles suddenly causing the leaves to fold up from loss of turgor. (I'm not saying that's how it happens, botanists, I'm just thinking out loud)

I am more intrigued by the possible reasons for such a behavior.

Could it be that closing your leaves when some herbivore touches you discourages the leaf muncher ? Is the plant hiding?

Could it be a way of kicking caterpillars off your leaves...as if someone pulled the rug out from under you?

I'm sure it's not happening...JUST BECAUSE.
There has to be a good survival reason for it.

What do you think?

(This is not a puzzler...except to me. Don't expect an answer from yours truly)



14 comments:

doubleknot said...

Has to be survival of some sort. I just used to like to touch them and watch them fold up.

Wayne said...

Oh yes, sensitive briar! I love it! I can entertain myself for hours playing with this poor plant.

And it's a great performer. Some of the other related species are disappointingly slow.

You have the mechanisms right, FC! There are little sensing organs at the base of the leaves whose name (pulvinus?) I don't remember that are involved. They do exactly what you say - they dump their potassium and the water follows by osmosis, and the leaflets "wilt", closing up.

The defense I've heard is that by closing up, the plant looks dead, and herbivores won't eat dead plants. But I love the idea of the action kicking off the caterpillars.

pablo said...

Glad to have you back from Grad Bash. I remember chaperoning those for many years in a row (four kids in high school at the same time!).

I sometimes wonder about seemingly pointless mechanisms in evolving plants and animals. Does every mechanism HAVE TO have a survival function? Couldn't something have evolved that is suvival neutral? It evolved. It doesn't really help in any way. It doesn't really hurt in any way. So the mechanism persists. I'm just guessing.

roger said...

the appendix of plant life? maybe the (relative) lack of water in the leaf makes it less palatable enough so animals will choose the juicy plant over there. i did not know those plants are briers. thanks.

Laura said...

There is definitely a learned survival mode that has emerged behind it all... think: BULLDOZERS. ;)

Floridacracker said...

Doubleknot,
I have poked a bunch of them too.

Wayne,
Thanks! I appreciate the "how it works" details. I knew this one would be right up your alley. Seems like "hiding" from herbivores is probably it.

Pablo,
4 in high school at the same time...whew. I won't have more than 2 in HS at any point, but that's busy enough.
As for the other thing...I still go with there's always a reason related to survival.

dpr,
these are not very briary, but their cousins have more substantial stickers.

Laura,
Okay, now I understand perfectly.
:)

Abandoned in Pasadena said...

I agree with you FC...there's always a reason for everything.

Tjilpi said...

Surely we can't admit that there are plants which may have evolved properties which are, essentially, useless?

What about Intelligent Design?

Surely there should be no such thing as useless stuff on the highway of evolution?

There has to be an intelligent reason for the behaviour of this plant!

[There is a similar plant which grows on the east coast of Queensland - great fun for kids.]

Pablo will get himself flagged by the monitors of cyberspace who watch for an apostasy such as the one he is proposing above.

Floridacracker said...

Abandoned,
Yup, what may appear happenstance is something we just haven't figured out yet.

Tjilpi,
Nope, every part and every behavior is there because it benefits survival of the species.

Evolution is the most intelligent design imaginable...only a superior being could have come up with it.
I see no conflict.

Wayne said...

It's not so common among evolutionary biologists who realize that so long as there isn't any *negative* selection, there need be no particular reason for many of the marvelous permutations on adaptations we see.

I think of the hairs (trichomes) on many related species of a particular genus - some are single hairs and many have just fabulous ornamentations.

An adaptationist, or an inquisitive layperson might try to find a reason why those hairs are different - "just so stories", but the simple explanation is that it doesn't matter - all of them work just fine and nothing in them makes the organism less likely to reproduce.

So there's all kinds of things that are hard to rationalize, not because we don't perceive a function that isn't that important, but because the structure doesn't negatively select against the organism.

It's kind of nice to me, since it results in a dazzling array of behavior and structures, without requiring that we need explain why everything is "just so".

As for where it all came from, it's as good a guess for anyone else as it is to me. Maybe.

Debbie said...

Thanks for the lesson everybody. When I was a child growing up in Miami my parents had a friend that had these plants in her yard. They entertained me for hours. When I asked the adults why the leaves folded up like that, they always said "magic". Now I realize that they just didn't know. I learn so much from your blog.

Floridacracker said...

Wayne,
I just practice what you once said, Announce things with an air of authority... I am paraphrasing.
:)

Debbie,
That is the wonderful thing about this blog world, a post can stimulate discussion and input from a host of different folks. I really like that part as much as writing the original posts.
I use the "magic" answer when the kids ask, "How can mom always find the stuff we can't find?"

Wayne said...

FC - I can't believe you remember that. That's pretty fantastic to me.

I'm going to have to remember the "magic" answer too.

Leslie said...

I like those little guys. Are they related to the mimosa? The mimosa is a tree but they have similar flowers and leaf structure (and the leaves also fold up when you mess with them).

I love mimosas!