Fluid Transport in vascular plants…Ed's summary. (Read at your peril)
Transpiration in the Xylem stream: You should be aware that the evaporation of water
from stomata creates a negative pressure that exerts a "pull" on water. Recall from earlier
in the year when we did the experiment with potatoes. Water moves from where its
potential ( "psi" pronounced "sigh") is greater to where it is lower. In that lab, we were
dealing only with the effect of solute concentration on total water potential ().
However, the actual formula for determining water potential is  = s + p. s is the
solute potential and its highest value is zero. As solute concentration goes up, s goes
down (increasingly negative). However, p (pressure potential) can be zero
(atomospheric pressure), positive, or negative. The evaporation of water from leaves
creates negative pressure, and therefore lowers the value of . Since water movement in
plants is ALWAYS from where  is higher to where it is lower, the negative pressure
resulting from evaporation at the stomata lowers  in that vicinity, and water is "pulled."
The collective negative pressure (suction) from evapotranspiration at millions of stomata
pulls the water upward. It took decades for scientists to figure this out, and not all of
them were convinced until the computer came along with its ability to calculate the
overall force resulting from evapotranspiration of water.
In stems and vascular bundles, there are two types of vessels through with Xylem "sap"
flows. Both are dead cells. Tracheids and vessels (=vessel elements).
Water movement in plants is passive in the sense that the plant does not use chemical
energy to move it. However, bringing ions into the roots from the soil involves active
transport (ion channels).
Translocation in the Phloem: As sugars and other compounds are made in the
chemically active parenchyma cells of leaves or in meristematic tissue, they are "loaded"
into the phloem to be transported from source to sink (where it is needed or where it will
be stored). To accomplish this loading, both passive and active transport are involved.
Phloem cells are alive and they are called sieve tube members (I understand "sieve" and
"tube" but MEMBER?). Whatever. The sieve tube members are loaded by companion
cells. These companion cells are between the sieve tube members and the bundle sheath
cells that surround the vascular bundle. After sugars and other compounds are actively
loaded into the sieve tube members, osmosis (passive) of water from neighboring xylem
vessels creates pressure in the sieve tube members and this gets the solutes in the phloem
sap moving in the direction of the sink. The process is called the pressure flow model. It
used to be called the pressure flow hypothesis, and still is in many books, but is pretty
much universally accepted by botanists these days.