North American Fruit Explorers mailing list at ibiblio

[Heron Breen <breen@fedcoseeds.com>]

Hey sports fans, 
The below address will take you to a wicked good website that highlights each 
part 
of the tree and describes its function. Useful and clear I hope.

http://hort.ifas.ufl.edu/woody/pruning/compartmentsaxial.htm

Yes so phloem gets pretty dense as time goes on in a trees life, with dead 
and 
interrupted sieve tubes, companion cells, parenchyma etc taking up space. The 
compressed nature of Phloem is not as evident in trees that are younger. 
Since 
phloem can sometimes be 1/2" or more in thickness in some species, it appears 
plenty thick in comparison with a few growth rings and thin bark. But as the 
years 
fly, its width increases slightly and its density grows. This density of 
crushed, 
dead, and living cells is theorized to be one of the many factors that can 
combine 
to slow grow (ie maturity) and cause woody plant death. Regardless, the 
cambium is 
always very thin and comparably the phloem is much thicker, ie one cell thick 
versus many cells thick. Although I have heard verbally that some of the dead 
cells 
within the Phloem may get "re-absorbed", I cannot confirm this and I do not 
suppose 
that this appreciably affects the cellular compression in mature and late 
mature 
stage. Phloem does not become cortex, that is clear, so no shedding in that 
way.
How does compression kill or further slow growth? A couple of ways, one being 
that 
existing seive tubes (the transporters of sugar solution down toward the 
roots) get 
crushed or closed of from injury and future tubes are smaller and able to 
carry 
less sugar volume. But the living storage cells still need to be fed, as well 
as 
the cambium. When the capacity to do this declines, the tree kills/walls off 
those 
layers of xylem that it can no longer feed, reducing living storage cells. 
Another 
words sapwood turns into heartwood. This also occurs when leave canopy 
reduces 
during maturity due to whatever cause. Canopy reduction and compression 
interplay 
in each condition's causation. The tree is trying to balance the energy in 
can 
produce and the energy it uses. There are a million details to these 
processes. 
Another factor is it takes energy to move sugar (energy) throughout the tree, 
it is 
not a passive process like water movement/transpiration.
So there is my cloudy attempt at a clear explanation.
sincerely,
Heron Breen
zone 4, Maine