North American Fruit Explorers mailing list at ibiblio

[Heron Breen <breen@fedcoseeds.com>]

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
>
>