[NAFEX] Phloem. Where does the phloem go?
breen at fedcoseeds.com
Sun Dec 11 21:38:09 EST 2005
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.
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.
zone 4, Maine
More information about the nafex