[NAFEX] Phloem. Where does the phloem go?
breen at fedcoseeds.com
Sun Dec 11 16:19:46 EST 2005
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,
zone 4, Maine
On Sun Dec 11 10:59 , Jwlehman at aol.com sent:
>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?Â Â Â
>Just trying to learn more.
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