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Transport of Water and Minerals in Plants





Link to Transpiration

However, transpiration does serve two useful functions:


Link to diagram of root structure.

The Pathway of Water


However, the inner boundary of the cortex, the endodermis, is impervious to water because of a band of lignified matrix called the casparian strip. Therefore, to enter the stele, apoplastic water must enter the symplasm of the endodermal cells. From here it can pass by plasmodesmata into the cells of the stele.





The Pathway of Minerals


They enter the water in the xylem from the cells of the pericycle (as well as of parenchyma cells surrounding the xylem) through specialized transmembrane channels.

What Forces Water Through the Xylem?





But even the best vacuum pump can pull water up to a height of only 34 ft (10.4 m) or so. This is because a column of water that high exerts a pressure of ~15 lb/in2 (103 kilopascals, kPa) just counterbalanced by the pressure of the atmosphere. How can water be drawn to the top of a sequoia (the tallest is 370 feet [113meters] high)? Taking all factors into account, a pull of at least 270 lb/in2 (~1.9 x 103 kPa) is probably needed.


Link to discussion of hydrogen bonding in water.

When ultrapure water is confined to tubes of very small bore, the force of cohesion between water molecules imparts great strength to the column of water. It has been reported that tensions as great as 3000 lb/in2 (21 x 103 kPa) are needed to break the column, about the value needed to break steel wires of the same diameter. In a sense, the cohesion of water molecules gives them the physical properties of solid wires.


Some support for the theory

Problems with the theory


So might cavitation break the column of water in the xylem and thus interrupt its flow? Probably not so long as the tension does not greatly exceed 270 lb/in2 (~1.9 x 103 kPa).

By spinning branches in a centrifuge, it has been shown that water in the xylem avoids cavitation at negative pressures exceeding 225 lb/in2 (~1.6 x 103 kPa). And the fact that sequoias can successfully lift water 358 ft (109 m) — which would require a tension of 270 lb/in2 (~1.9 x 103 kPa) — indicates that cavitation is avoided even at that value.

However, such heights may be approaching the limit for xylem transport. Measurements close to the top of the tallest living sequoia (370 ft [=113 m] high) show that the high tensions needed to get water up there have resulted in: (Reported by Koch, G. W. et al., in Nature, 22 April 2004.)

So the limits on water transport limit the ultimate height which trees can reach. The tallest tree ever measured, a Douglas fir, was 413 ft. (125.9 meters) high.

Root Pressure



that have been actively transported into the apoplast of the stele.




Link to discussion of food transport in the phloem.
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19 March 2014