Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 216

Fibre
cell

Sieve tube

Vessel

Vessel
member

Sieve tube
member

Bordered
pit

Sieve plate

Perforation
plate

Companion
cell

Va-
pour

Bubble of
water vapour

Nucleus

100 m m

Xylem Cambium Phloem

Figure 6.9 The vascular tissue of plants is composed of vessel tubes and sieve
tubes. The sieve tubes serve for transport of organic compounds such as assimilates,
whereas the vessel tubes convey water and minerals, as well as organic compounds,
metabolized in the roots. Arrows within the vessels indicate the direction of sap low.
The pit density is in reality many times the number shown in the diagram(Ehlers and
Goss, 2003 ). Reproduced with permission of CAB International, Wallingford, UK.

low path by transverse constrictions. Bordered pits are inserted into the strengthened,
ligniied longitudinal walls of the vessels, which are passage openings in a horizontal
direction ( Figure 6.9 ). They allow the transverse transport of water into neighbouring
cells, but may seal off the vessel by closing membranes, when by accident the vessel
has dried out (Ehlers and Goss, 2003 ).
In the vessels, water is conducted by mass low, like water through pipes, driven by
a gradient in hydraulic head. The long-distance transport is carried out at high speed.
The speed can be measured by heating the stem locally and measuring the tempera-
ture change at a point higher in the stem. In deciduous trees the velocity ranges from 1
to 40 m h -1 , and in herbaceous plants from 10 to 60 m h -1 . In the following paragraph
we calculate the hydraulic head gradient necessary to attain these low velocities.