P. van Rijckevorsel wrote:
"P. van Rijckevorsel" wrote

The answer is very simple when a tree just starts out, after germination,
all of its cells (100%) are alive. As it grows bigger and bigger, every
year the percentage of living cells drops. It only reaches zero when
the tree is truly and completely dead.


From: "Alan Meyer"

Thanks, but I wonder if it's really that simple.


If we plot a curve of living cells to total cells I bet we
wouldn't see anything approaching a straight line going
from 100% to 0% over the life of the tree.


***
No indeed, there will be nothing like a straight line.
Measuring every year at the same time will result in a inverse s-shaped
curve, broken at the end by whatever kills the tree.
* * *


I'd think we'd
see a couple of years at the beginning when all cells are
alive,


***
No. There will be dead cells pretty quickly
* * *


then a gradual decline as the tree grows up and out,
then perhaps a levelling off that lasts


***
basically till the end
* * *


for years as the
mature tree lives out its life, then a steady and perhaps
rapid decline.


***
That will depend entirely on what kills the tree
PvR
* * *


I'm particularly interested in that middle, mature period
when the tree has achieved its growth but is still very
viable.

Alan





Remember too, that unlike animal cells which are mostly alive, most of
the volume of "living" plant cells is also non-living space -- the cell
wall and middle lamella are non-living, as is the vacuole inside the
cell -- just a big bag of non-living liquid, so that perhaps 80-90% of
each living cell is actually non-living (part of the apoplast). The
symplast -- the truly living portion of the cell, consists just of
cytoplasm and its organelles, and is continuous throughout the plant via
plasmadesmota -- small tubes running across cell walls, linking the
cytoplasm and the plasma membranes of one cell to another. Also inside
a leaf, there is quite a lot of air space -- areas not occupied by
living cells. So if you consider all non-living space in the total
volume of a tree, you end up with quite a lot more than just xylem.