Growth limit

On 10/1/2010 8:47 AM, Greg D. Moore (Strider) wrote:


So I'd say it's sort of like the chicken and the egg. The DNA won't let
things grow to large because DNA that does gets selected against.

This is so cool; on lions the growth limiting DNA is in one sex, in
tigers the other, so if you crossbreed the two sexes that don't have the
growth limiting DNA, you come up with something that looks like it's
right out of the Pleistocene:
http://www.cryptomundo.com/wp-content/uploads/liger.jpg
I always wondered if the heights the sequoias and redwoods achieved was
due to the need to get their foliage above the grazing height of the
giant sauropods like Brachiosaurus...which when to think about it could
also explain their great width also, as one of those things rearing up
on its hind legs and leaning against a tree could probably knock over a
tree of pretty considerable size given its enormous weight, and get at
pretty high foliage that way also.

Pat

In article , LSMFT
wrote:

What keep plants and animals from growing out of all proportion in size.
Why not trees larger than the Sears tower. Animals larger that
Brontosaurus? Just DNA?

For tree height, the limit is set by the cohesive strength of the sap.

The sap in a tree's xylem flows is in continuous channels that reach
all the way to the root. It is pulled up the channels by water
evaporating in the leaves, which draws the water up. The roots only
supply a small amount of positive pressure to push it up, the rest is
just about all pull.

But you might ask how that is possible. Every ten meters of water in a
column makes 1 atmosphere of pressure in the column. So the sap is
going to be at zero absolute pressure about 10 meters above the ground.


That's why you can't use a pump at the top of a 10 m long tube to pull
water out of a tank or an open well with that deep a water table.

So how can you have a 110 meter sequoia?

The answer is that the sap is actually under tension most of the way.
If you suck on a liquid, it doesn't necessarily break apart when it
gets to zero pressure. It takes energy to form a surface (which is
equivalent to saying that there is surface tension) so letting the
water stretch under tension can be a lower energy state than having it
break.

If you put a pressure gauge in the sap at the top of a sequoia, you
will measure a -10 atmosphere pressure. But that is about the tensile
strength limit of water, and beyond this point the sap just breaks,
which is why you don't have trees much taller than sequoias.

There are ways that nature _could_ figure out to get taller trees (e.g.
multi-stage pumping), but that solution space seems to be unoccupied by
life so far on Earth.

--
David M. Palmer (formerly @clark.net, @ematic.com)