North American Fruit Explorers mailing list at ibiblio

[Heron Breen <breen@fedcoseeds.com>]

Hi sportsfans,
Offlist, Jerry Lehman asked me about how my previous posts about plant 
physiology 
relate to Maple Sugaring. I think explaining the conditions around the human 
production of maple syrup will continue the cycle of physiology we started. 
So here 
goes:

All summer long, the tree is transporting sugar solution down the phloem, and 
parenchyma cells are sucking as much as they can hold as the fluid descends. 
The 
solution is sugar and some water. The cambium layer is using a lot of this 
sugar to 
divide and create new Phloem parts and new xylem parts, as well as new 
"hungry" 
parenchyma cells within the xylem. Meanwhile water is "pumping" up from the 
roots 
to the leaves for transpiration/photosynethesis. 
When I previously posted, I seperated the dead water transport vessels in the 
xylem 
(wood) and the parenchyma sugar storage/transport cells. These cells do 
interact, 
actually, but it is good to keep them clear in your mind before melding them. 
As 
the sugar solution is transported into the wood, parenchyma and other cells, 
"fill" 
up and/or use some of the sugar and water for cell functions. Sometimes, if 
the 
tree is really going great and has lots of leaves and sun, the parenchyma 
can't 
store all the sugar and water at their disposal. This would be similar to our 
bodies only being able to absorb so much vitamin C and hour, regardless of 
intake. 
Many parenchyma cells abut the water transport vessels and "leftover" water 
regularly returns to the vessels for another journey to the top. Sometimes a 
small 
amount of leftover sugar gets released into the vessels as well. But this is 
a very 
very little amount of sugar and it gets put back into use by cambium up top. 
This 
description might encourage one to think of these vessels and parenchyma as 
able 
and readily willing to share their goods regularly. This is not the case. For 
more 
than 75% of the year, they are mostly seperate.
This changes in the Fall. Almost all the stored sugar is meant for the spring 
push 
of buds, flowers, leaves. Throughout the Fall, parenchyma are turning the 
sugars 
into starch (long-term storage form of sugar). A lot of this starch is 
"dumped" 
(see below paragraphs for "dumped" explanation) into the vessels and pumped 
by the 
water upward and outward. Buds formed that summer are given a massive amount 
of 
starch build-up in the wood behind the bud to draw from in the spring. 
Remember 
photosynthesis is barely happening during spring (a little from the cortex) 
and 
trees come out of dormancy and begin cambial activity from the top down, so 
the 
starch needs to be already in place for a bud to grow either leaf or flower. 
Most 
buds are on wood that have a limited number (sometimes only 1 or 2) of growth 
rings 
worth of xylem backing it up for fuel, so parenchyma from more thick xylem 
send 
theirs. This sugar release into the vessels from the parenchyma takes place 
after 
the leaves have fallen usually. This is why collecting scionwood and dormant 
cuttings in the fall can have variable results and must generally be done in 
late 
November into December at the earliest here in Maine. 
This brings us to maple syrup. Along with parenchyma and vessels, there are 
lots of 
fiber cells in the wood. They store water as well as adding the structural 
integrity of the tree. Fiber cells need to be dead to function structurally, 
and 
their walls are made up of criss=crossing layers of bundled cellulose (which 
is in 
turn bundled starch chains) and lignin which is the equivalent of tree 
lanolin. The 
cambium makes all these cells. The interior of the cell is called the lumen. 
The 
interior and exterior can store huge amounts of water, and this determines 
the 
weight of a piece of wood "dry" or "green". They fill and empty depending on 
many 
factors. After feeding the living cells around them, their excess water would 
also 
end up in the transport vessels.
These fiber cells go through the winter with water in their lumens 
(interiors). As 
it gets cold, this water freezes into pure crystals. In maple sugaring time, 
the 
cold nights followed by warm days freezes then thaws this water. As the water 
thaws, vapor/gas is produced. We've all seen this from ice cubes in the 
freezer and 
snowbanks. I do not honestly know why ice does this, so someone else will 
have to 
fill in here. Nor do I know what makes up the vapor. This vapor goes through 
the 
walls of the fiber cell into the rest of the xylem. There it exerts pressure 
on the 
rest of the cells present, the parenchyma and xylem. The parenchyma are 
squeezed 
and some more of their starch goes the only place in can: into the water 
transport 
vessels. The water transport vessels themselves are under pressure and, if we 
didn't tap the tree, this starch would go up to help buds prepare for winter. 
This 
process is going on all late fall and winter, as freezing and thawing take 
place 
within the tree. It is a fairly passive process, compared to phloem transport 
and 
photosynthesis. The starch stops travelling when it gets to the buds and 
collects 
there ready for Spring usage.
When we tap the tree, we are releasing that pressure. The tree, if punctured, 
is 
literally willing to squirt its "sap" out whatever whole. Tapping in the 
trunk 
yields more "sap" because more parenchyma cells and transport vessels are 
present 
due to thicker xylem compared to farther up the tree. This freezing and 
thawing 
period of time is the only time that enough starch is being dumped into the 
vessels 
for that solution to be called "sap". If a tree is tapped in the summer, it 
will 
not yield much if any syrup, just plain water with a drop of sugar per gallon 
probably.
So, my fingers are worn out and probably your eyes, so I'll stop there for 
now. 
Elaborate, question, and correct at your leisure. Pardon my sentence 
structure and 
spelling.
sincerely,
Heron Breen
zone 4, Maine
PS Next time: the short course on a tree going "dormant" from September 
onward, 
including the theories of why rootstocks influence early or late dormancy and 
winter hardiness. Maybe by next week after my fingers rest.