I made me pools of water, to water therewith the wood that bringeth forth trees... Ecclesiastes 2:6
"As the major component of trees, wood seems useful and even 
beautiful in
finished products, but scarcely biologically dramatic. Yet recent
research on the development of wood portrays an elaborate ballet which
proceeds inside young developing xylem cells.
This performance
culminates in their producing “molecular ‘tracks’ for the
cell-wall-producing machinery. This machinery moves along the
microtubules like an asphalt paver and continuously deposits wall
material on the outside of cells.
It is apparent that trees need a plumbing system to move water up
from the soil to the canopy, the most important part of which are the
leaves.
The leaves have to photosynthesize, and for this they need
carbon dioxide and water (as well as sunlight). Since carbon dioxide is a
gas, it has to diffuse into the leaves from the air. As water is scarce
up there, the leaves utilize a protective layer to prevent unnecessary
water loss. Thus, the leaves need controlled openings in leaf surfaces
to let in the air including carbon dioxide. This also however allows
water to evaporate and escape from inside the leaf.
evident that the plumbing design has to be very sophisticated indeed.
The piping must not collapse inward under the negative pressure. The
water conducting system (tissue) in plants is called xylem. In trees, a
new layer of xylem (called secondary xylem) is added in a ring around
the stem each year. Thus the tree progressively grows in diameter as
more and more wood is accumulated. The xylem largely consists of two
kinds of hollow conducting elements or cells.
The beauty of this is however that water molecules have a special feature called cohesion. They stick together through forces generated by hydrogen bonding.
--As water evaporates into the air from cells in the
leaf interior, the escaping molecules tug at the water molecules behind
them, and they tug at the ones farther into the plant.
This evaporation
process is called a Cohesion-Tension mechanism. It creates a
negative pressure which holds the water column together under tension,
pulling it up through the plant’s woody plumbing all the way from the
root tips far below. Thus, as water escapes from photosynthesizing
leaves, a steady column of water is drawn up from the soil into the
canopy.
--Since the water column is held in such tension inside trees, it is
--The smaller ones are
tracheids,
--the larger are called vessels.
Tracheids are shorter in
length and of smaller diameter than vessels. These smaller elements are
connected with cells above and below by means of overlapping tapering
ends and numerous border pits.
Vessels are much larger cells. They too have border pits in the side
walls, They are stacked end to end with ends entirely open or with
perforation plates.
--Vessels exhibit diameters approximately that of a
human hair. These water conduits may be about 2 inches long or as
long as 33 ft. The water moves upward through the tracheids and
vessels, and sideways through border pits to new vessels and tracheids
as upward progress through individual extensions ends.
A future xylem vessel or tracheid initially is an ordinary living cell
equipped with all the usual interior organelles. 
The outer boundary of
the cell is a typical plasma membrane made up of a phospholipid bilayer
with various proteins situated in the expanse.
--Lying immediately below
the plasma membrane and parallel to it are microtubules. These tiny
protein tubes are dynamic structures, always forming new material at the
front end and shrinking at the back end. Thus, the tubules are
continuously advancing forward under the plasma membrane....Penetrating through the plasma membrane from the cell interior to its
exterior are very large protein structures called cellulose synthetic
complex (CSC). According to an article on these structures, current
estimates suggest that this is “one of the largest protein complexes”
that we 
know about.
--Electron micrograph images reveal six hexagonally
arranged particles arranged on the surface like a rosette, but they are
really rectangular molecules that penetrate the plasma membrane from
cell interior to the outside.
Closer examination however reveals that
these are elaborate structures: Each of the six lobes of the rosette in turn consists
of six cellulose synthases [enzymes expediting formation of cellulose],
that each polymerizes a single glucan chain using UDP-glucose as a
substrate.
--Those individual chains are then assembled into one
crystalline CMF [cellulose microfibril], which by implication consists
of 6 X 6 = 36 chains [of cellulose].
The microtubules inside the xylem cell and the rosette CSCs embedded in the plasma membrane, together cooperate to produce strong walls that are deposited in intricate patterns taking the form of rings, spirals, nets, and pitted patterns. But how do the microtubules know how to guide the machinery depositing cellulose outside the cell wall? As long ago as 1975 one specialist suggested:
It is proposed that plasmalemma [plasma membrane] located cellulose synthase enzyme complexes are free to move in the plane of the membrane. Their directed movement may …. generate a sliding force which moves the entire complex through the membrane utilizing the microtubule as a rigid guiding track and thus laying down, in the wake of the complex, cellulose fibrils whose orientation mirrors that of the microtubules.
How amazing is that!
Here are large molecular machines able to move
through the plasma membrane!
And even more confounding to our
imaginations is the idea of microtubules arranging themselves into
various elaborate patterns in order to guide the depositing of cellulose
outside the cell.
It is just recently that scientists from the
Netherlands and Germany have described the elaborate dance of the
microtubules which leads to the fancy xylem wall patterns.
Q: How do they do it?
A: You have probably seen dance moves involving two
steps forward, one step back, or one step forward and two steps back.
Well microtubules exhibit similar patterns of motion which are no less
an art form.
Young xylem cells start out with microtubules evenly arranged in
multiple directions parallel to the surface of the plasma membrane. The
CSC are already arranged in the membrane above the microtubules and a
thin primary cellulose wall is deposited with cellulose fibrils randomly
arranged like the orientation of the microtubules below.
But
microtubules are dynamic. While the initial scene finds the
microtubules evenly dispersed and facing in multiple directions, they
end up all oriented in the same direction with gaps between bands of
microtubules. The result is that the microtubules “directionally and
spatially template the cellulose synthesis machinery during cell wall
deposition.”
This process has to be directed and coordinated. Microtubules are constantly on the move.
However, this is an elaborate
progression involving “catastrophes” and “rescues”. Catastrophes
represent the occasion when forward growth of a tubule actually stops
and disintegration of the tip (depolymerization) takes place. Rescues
result in forward growth starting again.
In plants, there are signaling proteins which serve to recruit other
proteins to regulate microtubule advance or retreat. Of course, this
just puts the question of control one step farther back, what controls
the signaling proteins.
The nature of trees and their major impact on life on Earth depends upon
their ability to manufacture woody stems. The ability of plants to
funnel certain cells into the wood developing process, is as important
as are the amazing processes that lead to the appearance of wood itself."
By Margaret Helder CEH