This has puzzled me for a while.

If you're trying to identify a particular plant, often the most 
diagnostic feature is the leaf shape. And that's because plants have a 
dazzling array of leaf shapes. For example,

http://upload.wikimedia.org/wikipedia/commons/e/e8/Leaf_morphology.svg

The fact that humans even needed to invent this many words to describe 
leaf shapes tells you that there's a hell of a lot of leaf shapes out there.

This is puzzling because leaves all do the exact same job. (Well, the 
leaves that haven't been turned into petals or insulation or fly traps 
or some other specialised structure, anyway.) Given that all leaves 
perform the same task, you would expect convergent evolution to result 
in identical leaves. But this hasn't happened. Quite the opposite, in fact.

In particular, the purpose of leaves is to absorb as much sunlight as 
possible. Given that task, you would /expect/ leaves to evolve to be 
huge flat sheets. You would /not/ expect leaves to evolve holes and cuts 
in them. You certainly wouldn't expect to see long, thin, pointy leaves 
which hardly soak up any light at all.

But plants of course are not designed by human thinking. They are 
designed by a far more reliable process: trial and error. The vast 
number of plants with weird shaped leaves demonstrates beyond question 
that this /works/, and that it provides some sort of advantage. But... what?

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On 21-5-2012 12:51, Invisible wrote:

> In particular, the purpose of leaves is to absorb as much sunlight as
> possible. Given that task, you would /expect/ leaves to evolve to be
> huge flat sheets.

Huge flat sheets would mean that only the outher layers get sunlight. 
Inner branches might just as well commit suicide immediately.



-- 
tip: do not run in an unknown place when it is too dark to see the 
floor, unless you prefer to not use uppercase.

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andrel <byt### [at] gmailcom> wrote:
> On 21-5-2012 12:51, Invisible wrote:

> > In particular, the purpose of leaves is to absorb as much sunlight as
> > possible. Given that task, you would /expect/ leaves to evolve to be
> > huge flat sheets.

> Huge flat sheets would mean that only the outher layers get sunlight. 
> Inner branches might just as well commit suicide immediately.

  From an evolutionary point of view, trees are the result of an
evolutionary arms race.

  Tree trunks are basically a huge waste or resources. A tree doesn't
benefit much from an enormous trunk of wood; it doesn't serve much
biological purpose. The only reason why trees have such enormous wastes
of resources is to simply get its leaves higher than the competition.

  If all the trees in a forest were like 1 meter high, that would be
enough. However, immediately when one of the trees gets a slight mutation
that makes it grow a bit taller than the rest, it gets a survival advantage
while at the same time lessening the survival chances of the other trees
around it (because now the taller tree is blocking the sun for them for
a period of time). One such tree might not do much damage to others, but
when it reproduces and its offspring start shadowing other trees en masse,
those other trees are going to have a hard time getting any sunlight at all.
Thus the tallest of those other tree species are going to get naturally
selected from among their shorter brethren, and so on. Thus we end up with
gigantic trees which may reach even 100 meters.

  Dense forests with tall trees tend to be relatively barren of other
plant life, save for plants that survive on low light conditions. Something
like this: http://i304.photobucket.com/albums/nn198/toshi0723/CIMG1966.jpg
(Compare to something like
http://en.wikipedia.org/wiki/File:Hokkaido,_Sarobetsu_Plain.jpg )

-- 
                                                          - Warp

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>> In particular, the purpose of leaves is to absorb as much sunlight as
>> possible. Given that task, you would /expect/ leaves to evolve to be
>> huge flat sheets.
>
> Huge flat sheets would mean that only the outher layers get sunlight.
> Inner branches might just as well commit suicide immediately.

Exhibit A:
http://momsrecipes.nielsonpi.com/images113/Rhubarb3.jpg

Exhibit B:
http://i.pbase.com/u38/kayakbiker/upload/31926440.BananaLeafUnbrella.jpg

Exhibit C:
http://www.vagabondjourney.com/2008-1/08-2208-large-leaf-costa-rica.jpg

Clearly big leaves are not a suicidal adaptation...

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On 21/05/2012 10:23 PM, Warp wrote:

>     From an evolutionary point of view, trees are the result of an
> evolutionary arms race.
>
>    Tree trunks are basically a huge waste or resources. A tree doesn't
> benefit much from an enormous trunk of wood; it doesn't serve much
> biological purpose. The only reason why trees have such enormous wastes
> of resources is to simply get its leaves higher than the competition.

Well, I guess that's true enough.

Still doesn't explain why every type of tree has completely different 
leaves. Nor why it grows thousands of tiny ones rather than several big 
ones. (Although, I guess it means you can replace the damaged ones faster?)

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>> In particular, the purpose of leaves is to absorb as much sunlight as
>> possible. Given that task, you would /expect/ leaves to evolve to be
>> huge flat sheets.
>
> Huge flat sheets would mean that only the outher layers get sunlight.
> Inner branches might just as well commit suicide immediately.

Huge flat sheets are not a good design for several reasons:

1) Which orientation would you align these huge leaves towards?  Or you 
mean to arrange them like a balloon covering all directions with a 
hollow inside?  In both cases your tree is going to be a huge sail and 
get uprooted very easily in strong winds.

2) They would need to be much thicker (and heavier) than lots of smaller 
leaves in order to keep their shape, and thus the branches supporting 
them would need to be much thicker (and heavier).  As warp already 
mentioned, other trees would then be able to get higher with more 
efficient designs (thinner branches and more smaller leaves) and at 
least partially block your sunlight.

3) Leaves fail all the time, they get eaten, blown off, snapped off etc. 
  If you've only got a few large sheets then there isn't much redundancy 
in the system, your tree will likely die if just 1 or 2 leaves get broken.

4) With lots of small leaves it is easy for the tree to adjust to 
maximise the sunlight, it can let a few leaves die that are in shade and 
grow news ones to take advantage of extra light very quickly.  With a 
small number of huge leaves that is not so easy.

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On 22-5-2012 10:02, Invisible wrote:
>>> In particular, the purpose of leaves is to absorb as much sunlight as
>>> possible. Given that task, you would /expect/ leaves to evolve to be
>>> huge flat sheets.
>>
>> Huge flat sheets would mean that only the outher layers get sunlight.
>> Inner branches might just as well commit suicide immediately.
>
> Exhibit A:
> http://momsrecipes.nielsonpi.com/images113/Rhubarb3.jpg
>
> Exhibit B:
> http://i.pbase.com/u38/kayakbiker/upload/31926440.BananaLeafUnbrella.jpg
>
> Exhibit C:
> http://www.vagabondjourney.com/2008-1/08-2208-large-leaf-costa-rica.jpg
>
> Clearly big leaves are not a suicidal adaptation...

Did you notice the branches on the inside with the large leaves? Me 
neither.


-- 
tip: do not run in an unknown place when it is too dark to see the 
floor, unless you prefer to not use uppercase.

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Le 22/05/2012 10:07, scott a écrit :

> 3) Leaves fail all the time, they get eaten, blown off, snapped off etc.
>  If you've only got a few large sheets then there isn't much redundancy
> in the system, your tree will likely die if just 1 or 2 leaves get broken.

hint about Rhubarb (Exhibit A from Invisible): the leaf is toxic (as is
the root). This greatly reduces the impact of being eaten.

(the branch is edible for human, stay away from leaf & root!).

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> hint about Rhubarb (Exhibit A from Invisible): the leaf is toxic (as is
> the root). This greatly reduces the impact of being eaten.
>
> (the branch is edible for human, stay away from leaf&  root!).

Good to know!

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Le_Forgeron <lef### [at] freefr> wrote:
> hint about Rhubarb (Exhibit A from Invisible): the leaf is toxic (as is
> the root). This greatly reduces the impact of being eaten.
>
> (the branch is edible for human, stay away from leaf & root!).

More than edible, it's downright tasty!
:D

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