The plates on the following paper show clearly the size of xylem and number of xylem in comparison to phloem. Tiny amount of return flow in the phloem = solvent dragging on all of the more dilute sap in the xylem, evaporation at the leaves reduces the volume of sap but increases the density of the sap and the denser sap moves down the tiny by comparison phloem vessels.
Experiments with different diameter tubes has shown this to be worth considering.
Using an inverted U tube with 3 tubes one of which contained a small amount of coloured salt solution added at the upper part where the tubes were joined with a T junction, the salt solution moved down as expected and the salt free water in the other two tubes moved up, caused by solvent dragging on all of the water molecules.
In the case of the tree, the narrowing of the xylem vessels in the canopy compared to the xylem vessels in the trunk and branches affords a method of extruding the large volume of water shedding off the majority to evaporation and returning the solutes down the phloem vessels.
The paper also relates to the problem with addressing constant cavitations known to take place and also known to refill and repair the cavitations. The cohesion tension theory relies on root pressure for this, yet the rattan does not exhibit root pressure.
www.amjbot.org/cgi/reprint/89/2/196.pdf
American Journal of Botany 89(2): 196–202. 2002.
XYLEM OF RATTANS: VESSEL DIMENSIONS IN
CLIMBING PALMS1
JACK B. FISHER,2,3,5 HUGH T. W. TAN,4 AND LESLIE P. L. TOH4
environmental factors. During periods of limited rainfall, rattans
and other lianas can experience severe water stress. At
such times, both stomatal closure and stem water storage
would aid survival. In other lianas, water-storing tubers or succulence
of stems and leaves are common (Fisher and Ewers,
1991). Rattans lack tuberous roots and their narrow stems have
a small proportion of parenchyma that could function in water
storage. However, their long stems with a relatively large volume
of water in wide vessels represent a significant water reservoir
that would become available if cavitation of vessels
occurred during periods of extreme water stress (Holbrook,
1995). If cavitation of wide vessels does play a role in water
supply during draught periods, then the question of vessel refilling
must be addressed. Further studies should also focus on
the water capacity of rattan stems compared to nearby nonclimbing
palms, as well as their relative degrees of stomatal
control.
At present, we have no information on production of embolisms
in rattan xylem. Yet the low percentage of nonfunctional
vascular bundles in old stems suggests either a lack of
vessel cavitation or a mechanism for refilling vessels (and tracheids).
Other lianas have root pressure that is sufficient to
refill air-filled xylem, as in Vitis (Sperry et al., 1987), or to
decrease xylem tension and thus assist in removal of embolisms
(Fisher et al., 1997). In a nonclimbing palm, Sperry
(1986) found that embolisms were dissolved when xylem pressure
potential approached that of the atmosphere during periods
of rain. When stem bases of cultivated species of Calamus,
Daemonorops, and Desmoncus (a climbing nonrattan palm)
were cut at dawn during rainy periods, no exudation appeared,
thus indicating no root pressure (Fisher et al., 1997); however,
there was an indication of root pressure in one species of Calamus
cultivated in a mountainous rainforest. We suggest that
future measurements for possible root pressure are needed to
better understand water conduction for rattans growing in natural
environments.
environmental factors. During periods of limited rainfall, rattans
and other lianas can experience severe water stress. At
such times, both stomatal closure and stem water storage
would aid survival. In other lianas, water-storing tubers or succulence
of stems and leaves are common (Fisher and Ewers,
1991). Rattans lack tuberous roots and their narrow stems have
a small proportion of parenchyma that could function in water
storage. However, their long stems with a relatively large volume
of water in wide vessels represent a significant water reservoir
that would become available if cavitation of vessels
occurred during periods of extreme water stress (Holbrook,
1995). If cavitation of wide vessels does play a role in water
supply during draught periods, then the question of vessel refilling
must be addressed. Further studies should also focus on
the water capacity of rattan stems compared to nearby nonclimbing
palms, as well as their relative degrees of stomatal
control.
At present, we have no information on production of embolisms
in rattan xylem. Yet the low percentage of nonfunctional
vascular bundles in old stems suggests either a lack of
vessel cavitation or a mechanism for refilling vessels (and tracheids).
Other lianas have root pressure that is sufficient to
refill air-filled xylem, as in Vitis (Sperry et al., 1987), or to
decrease xylem tension and thus assist in removal of embolisms
(Fisher et al., 1997). In a nonclimbing palm, Sperry
(1986) found that embolisms were dissolved when xylem pressure
potential approached that of the atmosphere during periods
of rain. When stem bases of cultivated species of Calamus,
Daemonorops, and Desmoncus (a climbing nonrattan palm)
were cut at dawn during rainy periods, no exudation appeared,
thus indicating no root pressure (Fisher et al., 1997); however,
there was an indication of root pressure in one species of Calamus
cultivated in a mountainous rainforest. We suggest that
future measurements for possible root pressure are needed to
better understand water conduction for rattans growing in natural
environments.
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