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Around the country park at the back of my house, there are several very tall trees (20m high plus) which have lost most of their leaves over the last few days. My daughter asked why the leaves on the top metre or so are still there, and I couldn't give her an answer. I don't even know what sort of trees they are, which is a pretty poor show I know. But why should the leaves on the most wind-blown part stay put the longest?
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Because they are the youngest, fittest and smallest. Plus the branches are the ones that flex the most allowing the leaves to trail in the wind.
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That makes sense, thanks. I had an idea they would be more resilient because they're more exposed, natural selection kinda thing.
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I'd noticed same phenomenon from train window; agree Zero's answer makes sense.
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If you can describe the leaves we can probably ID the tree.... my guess, in an urban setting, would be either Sycamore (multi pointed leaves, quite fat) or Lime (rounder leaves with a single point, and rather sticky.
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But won't the leaves near the end of all the branches be the youngest, not just those at the top?
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My guess is that the leaves furthest from the base of the tree, top or end of all branches, are the last to get the message that says "Die and Fall Off!"
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"My guess is that the leaves furthest from the base of the tree, top or
end of all branches, are the last to get the message that says "Die and
Fall Off!"
That would be my guess too but of course trees don't have a central nervous system, rather the signal to close down is generated locally within the leaves and is triggered by day-length/light level.
I suspect that the unshaded upper leaves are the last to reach the critical light level that triggers leaf fall
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>> but of course trees don't have a central nervous
>> system, >>
Presumably not. But what is it that causes the entire tree to grow in a characteristic tree shape, rather than each branch simply fighting upwards for light?
If you grow a collection of trees too close together, all the trunks grow vertically, tall and abnormally thin. yet a collection of branches on the same tree seem to "know" which is the main trunk and which are designed only to be branches.
Or again, if you cut off the end of a side branch that was growing horizontally and plant it on its own so that it roots, the branch turns into a trunk and starts growing vertically.
And different kinds of tree grow in characteristically different shapes. How does any one bit know how to grow, so that the overall shape is correct?
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Interesting question. The growing point of a tree, or indeed any plant produces a hormone that suppresses growth form the lower growth points (axillary buds). Remove the main growing point from a tree by pruning and the hormone supply will cease and growth will start from an axillary bud which itself will itself start producing hormone. The strongest growing branch will eventually take over as the main point of growth.
This feature is often used by gardeners pinching out the growth point when wanting a bushy plant. .
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>> Or again, if you cut off the end of a side branch that was growing horizontally and plant it on its own so that it roots, the branch turns intoa trunk and starts growing vertically.
Most plants exhibit the phenomena of positive geotropism ( they grow upwards towards the sky and downwards towards the earth ) and positive phototropism ( they grow towards light ) in order to maximise their nutrition intake on the one hand and their opportunity to be best placed for the process of photosynthesis on the other.
Well I'm impressed even if you lot aren't....been 35 years since I did Higher Biology...
Anyway, I've got a squirrel theory...Yer squirrel only goes on branches which will take his weight. Thin branches would break. Therefore he knocks the dead leaves off the thicker branches doesn't he while he's scurrying about looking for nuts...The thin branches near the top and at the ends of the lower spurs don't get the squirrel interference..
Well it might be true !
Last edited by: Humph D'bout on Thu 4 Nov 10 at 12:58
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>> Most plants exhibit the phenomena of positive geotropism
Funnily enough only a couple of weeks ago I set to trying to work out how geotropism actually works. If you search for it, you tend to end up with answers of the "it works due to gravity" variety or some such self-referential nonsense. The actual mechanism turned out to be quite interesting, as did the life story of the first proponent.
That lead inevitably, of course, to Big Questions like "how does gravity work", which has always been a mystery to me. I'm glad to discover there isn't really a definitive answer to that as yet.
I don't know why I typed this now. My memory isn't..big grey thing..sieve..burble.
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>> I don't know why I typed this now. My memory isn't..big grey thing..sieve..burble.
I know what you mean. I read the original post on this last night and ended up lying in bed trying to work out the answer. The squirrel thing for example came to me about 03.30 so it's not to be trusted...
Ray Mears must know the truth.
:-)
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>>Ray Mears must know the truth.
Ray Mears is a self-aggrandizing know-it-all in my opinion. Not a patch on Les Hiddens.
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Ray Mears is my hero. The One Show hasn't been the same since he and that Irish bird left....
:-0
( I do know it's Adrian Chiles...)
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Humph, Robert Llewellin was on telly this morning. I think he's a fantastic presenter, but did you know he started off making hand-made shoes, in fact I think he said he worked for Lobbs. He had a pair with him that he'd made. And he repaired Frank Sinatra's shoes once, but wasn't allowed near Charles's polo shoes.
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That's like saying your fingers are older than your body.
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Only roots exhibit positive geotropism, whilst branches exhibit negative geotropism.
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S'pose...maybe...technically that might be right...a bit...
:-)
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>> Only roots exhibit positive geotropism, whilst branches exhibit negative geotropism.
Which means all branches would grow upwards? they don't and grow in parallel to gravity, so what kind of geotropism is that?
and how would they grow in space in negative G?
Last edited by: Zero on Thu 4 Nov 10 at 15:38
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>> and how would they grow in space in negative G?
>>
Every schoolchild knows that...
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Ah but, now, y'see, thing is, only the roots are exclusively affected by geotropism and of course soil type and content. The branches conversely are affected by geotropism, phototropism, climate and habitat ( and squirrels )
No squirrels in space y'see so its going to skew the experiment, mark my words...
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>> No squirrels in space y'see...
Say what?
www.youtube.com/watch?v=lnUkkilqg9c
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The Muppets had "Pigs in Space" so why not squirrels?
I think that's very speciesistic (?) :-)
John
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Our Beech tress are shedding now at an alarming rate which makes finding the Labrador doo quite difficult.........................................But persevere I told her, persevere. Then she hit me!
Think I'll buy her a new Shat Nav for Christmas.
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Tree question - Zero
