Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 75

The phloem predominately moves fluids downward from
plant-produced dissolved organic substances in the leaves to
the roots. The driving force for this movement of sap is the
concentration gradient that exists between the zone of pro-
duction in the leaves and the zone of consumption. Because
the concentration of sugars is high in the leaves, the concen-
tration of water is low. Water enters from the xylem to dilute
this solution and creates high osmotic pressures in the leaf
phloem. Conversely, in the roots these sugars are oxidized
for growth, and the phloem concentration is rendered
diluted, which causes water to exit these cells and create a
decrease in osmotic pressure (Fig. 3.13 ). These relations lead
to the downward movement of sap from the leaves to the
roots. The direction can be reversed during the spring in
deciduous trees, such that the roots contain more sugar
than the new leaves and the sap flows up the phloem, called
the pressure-flow hypothesis. However, some sap flow is not
controlled by either pressure or gravity, as some conifers
yield sap during daylight to heal wounds.

3.5.1.3 Law of Laplace
The diameter of the conducting vessels in vascular plants is
very small, from the root hairs to the xylem. These cell walls
must be thick enough, however, to withstand the strain when
these vessels are under tension or pressure, but remain thin
enough to ensure intracellular exchange by diffusion. The
existence of such an apparent contradiction is best explained
in terms of the relation between the radius of a vessel and its
wall strength, called the Law of Laplace:

T ¼ PR ; (3.6)

where T is the tension on the vascular wall (dynes/cm 2 ), P is
the pressure (mm Hg), and R is the radius of the vessel (cm).
If the radius is large, the wall thickness of the vascular tube
also must be large; if the radius is small, the wall thickness is
small. This explains how even in the largest trees, the vas-
cular system can be composed of many narrow, thin-walled
tubes.

3.5.1.4 Rays
As we have seen from the previous discussion of the cellular
components of the plant tissues, most are vertical in orienta-
tion because they arise from the vertically growing cam-
bium. Some horizontal tissues also can arise, however.
Lateral movement of fluid from the outer rings of phloem
to the inner non-living xylem tissues can occur through pith
rays. A ray consists of parenchymal cells produced by the
cambium. In softwoods, some ray cells have ducts that carry
resinous compounds. Rays can be present in young plants as
a result of primary growth of the terminal meristem of the
bud and function to store or translocate food.

3.5.1.5 Ring and Diffuse Porosity
The location, size, and number of xylem vessels are charac-
teristically different for trees based on the presence of water-
conducting structures of the vessels, tracheids, fibers, and
parenchymal cells. In coniferous plants, the xylem does not
contain true vessels, and such trees are called non-porous
trees. In deciduous plants, the wood is characterized by
vessels having different structural appearances. These
trees, called diffuse-porous trees, have both tracheids and
vessels and a more uniform distribution within each annual
ring, where the vessels are smaller in radius and farther apart
from each other. Conversely, ring-porous trees have vessels
distributed in the early wood and are larger and more closely
spaced. In diffuse-porous trees, water can move in the
vessels in multiple rings; in poplar trees, for example,
water movement has been observed to occur as deeply as
4-8 cm into a tree. In ring-porous trees,

Fig. 3.13 Advancement of an increment corer (shown as the
rectangles ) into a tree reveals a horizontal pressure gradient, such that
the phloem is pressurized (+P) and the xylem is often under tension
( P). If the older xylem is decayed, it is possible that it also is
pressurized, however, and can emit gases, such as methane produced
by methanogenic bacteria in the rotting, anoxic wood.

the water is

conducted only in the outer annual ring.