[NAFEX] Phloem. Where does the phloem go?

Heron Breen 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,
Heron Breen
zone 4, Maine 

On Sun Dec 11 10:59 , Jwlehman at 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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