Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 219

together by high intermolecular binding forces. A pulling force of more than 300 bar
is necessary to separate water molecules from each other. Even in the highest trees the
cohesion between water molecules should be suficiently high to ensure intactness of
the water threads within the vessels. But in reality the linkage can be broken by cavi-
tation. The formation of 'gas seeds' breaks the water threads immediately.
Plants will protect themselves from disastrous consequences of cavitation by
partial breaks of the conducting vessel tubes. These take the form of open per-
forations or perforation plates ( Figure 6.9 ). Plants cannot avert the onset of cav-
itation. As soon as the water thread in a vessel is broken, the water low will be
diverted into neighbouring vessels by the bordered pits of a vessel below and above
the obstructed vessel member ( Figure 6.9 ). At the same time the expansion of the
bubble from one vessel to the next is limited by the speciic vessel structure. The
holes at the perforations and the pits at the walls are good for liquid but not for gas
transport. Because of the surface tension of water at the liquid-vapour interface,
the holes will stop the air bubbles from escaping or steadily enlarging. During the
night, when the transpiration rate falls to zero and the tension eases, the air bubbles
can be dissolved again. Thus the damage is repaired and the diversion will be closed
(Ehlers and Goss, 2003 ).
6.4 Transpiration, Photosynthesis and Stomatal Control
6.4.1 Transpiration
Plants take up liquid water, with nutrients dissolved in it, from the soil through their
roots. The water is transported upward and most of it leaves the plant, as water vapour,
whereas only a small fraction (about 1%) is used in the photosynthesis process. Los-
ing this water is the unavoidable by-product of carbon exchange.
The water vapour leaves the plants through the stomata. These are small openings
that occur mainly on the plant leaves, but to a lesser extent on other plant organs as
well. Stomata are the main path way for the exchange of both CO 2 and water vapour
between the plant and the atmosphere because the cuticle is rather impermeable for
gases ( Figure 6.11 ). In herbaceous plants stomata occur at both the upper and the
lower side of leaves, whereas trees have stomata only at the lower side (Willmer and
Fricker, 1996 ). The density and size of stomata varies considerably between plant
species, but there is a roughly inverse relationship between stomatal density and
stomate size that leads to a rather constant total area of stomata (Hetherington and
Woodward, 2003 ). Typical stomatal densities are 200 per mm 2 (but ranging from 50
to 1000 per mm 2 ). The length of the guard cells, which regulate the stomatal open-
ing, ranges from 20 to 80 μ m. With a typical stomatal aperture of 6 μ m, a fully open
stomate has a pore area of about 2·10 -4 mm 2 (Franks and Beerling, 2009). The total
area of the pores, when open, amounts to about 2-5% of the total leaf area (Willmer
and Fricker, 1996 ).