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  • From: Heron Breen <breen@fedcoseeds.com>
  • To: North American Fruit Explorers <nafex@lists.ibiblio.org>
  • Subject: Re: [NAFEX] Phloem. Where does the phloem go?
  • Date: Sun, 11 Dec 2005 15:19:46 -0600

Hi Jerry and others,
I will write a more detailed explanation later, unless someone beats me to
it, but
the short answer is: in general phloem gets squished between the cortex and
the
cambuim/xylem. Yes it has room to expand some as more pathways are laid down
due to
the cortex forming bark and the bark shedding, but the factors determining
the size
of the phloem and how mcuh room it has depends on the species and age of a
tree.
Viewing the individual parts as connected is where another key joining
element
comes in: the parenchyma cells.
Parenchyma cells transport sugars solution from the phloem for into the xylem
for
storage. Parenchyma start in the phloem (some extensions into the cortex as
the
cortex does photosynthesis too) and extend through the cambium, and are the
main
supplier of sugar solution as food to the cambium. Then they continue on into
the
xylem to be stored until needed, mostly used for the big push in spring.
Parenchyma
are also called rays and their are primarily two forms (with some
specialization
within each form) called radial and axiallary. These names denote their
orientation
within the xylem/wood. Parenchyma cells are seperate from the water transport
vessels also in the xylem, although they do abut and "do business" with each
other.
But the key thing to remember is the water transport vessels are dead and
parenchyma are alive. Phsyiology experts debate whether to label the
parenchyma as
part of the phloem or xylem, or as it own part. Many students I have talked
to in
our university hort progam in Maine have been taught that parenchyma are
extensions
of the phloem into the xylem. New storage pathways and cells are laid down
yearly
and can be used and accessed for many years even through the growth ring
layer.
got to go,
Heron Breen
zone 4, Maine


On Sun Dec 11 10:59 , Jwlehman@aol.com sent:

>Dear Heron,
>
>
>
>I'm going to let my ignorance hang out here and hopefully I'm not the only
>reader
with this question.
>
>
>
>Correct me here if I'm wrong. The vascular cambium lies between the xylem
>(inside)
and phloem (outside). Cambium cells divide in three directions with the
inside
cells becoming xylem and the outside cells becoming phloem. The third
direction is
sideways allowing the cambium ring to expand and push out as xzlem is formed
inside
pushing this cambium ring out. Outside of the phloem is a cork cambium which
grows
cork which becomes bark.
>
>
>
>My question is likely because there is a fine point in the above I'm
>missing. What
becomes of the phloem ring as the vascular cambium ring grows outward. Where
do
they go or stated in another way, what happens to those phloem cells? Are the
phloem cells incorporated into the cambium? Are they pushed outward and
convert to
cord cambium?   
>
>
>
>Just trying to learn more.
>
>
>
>Jerry Lehman
>
>





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