How does water really reach the leaves of trees?

Andrew Kenneth Fletcher gravitystudy at
Mon Feb 28 19:16:20 EST 2000

Hi everyone

Could we re-open this topic for discussion? I feel I have something new to
add to this field and wander if others find the accepted explanations for
fluid transport somewhat confusing.

Has anyone developed a working model which demonstrates a lift of water
higher that the 10 metre limit set down in the physics literature some three
hundred years ago.

I have read about osmosis, capillary action and root pressure, but find them
lacking in scientific validity.

Has anyone heard of alternative theories and if so could you provide us with
the location to start this thread.

Andrew K Fletcher

"I have chosen to relate to the following text book because it is written by
a person who like myself is not entirely satisfied by the explanations put
forward in the relevant subjects".

For the currently accepted view of osmosis and other views on water
transport I will refer to one of the standard GCSE text books entitled GCSE
BIOLOGY, D.G. Mackean. ISBN 0-7195-4281-2 first published in 1986.


Osmosis is the special name used to describe the diffusion of water across a
membrane, from a dilute solution to a more concentrated solution. In biology
this usually means the diffusion of water into or out of cells Osmosis is
just one special kind of because it is only water molecules and their
movement we are considering. Figure 3 showed that molecules will diffuse
from a region where there are a lot of them to a region where they are fewer
in number; that is from a region of highly concentrated molecules to a
region of lower concentration. Pure water has the highest possible
concentration of water molecules; it is 100% water molecules, all of them
free to move.

Figure 9 shows a concentrated sugar solution, separated from a dilute
solution by a membrane, which allows water molecules to pass through. The
dilute solution, in effect contains more water molecules than the
concentrated solution. As a result of this difference in concentration,
water molecules will diffuse from the dilute to the concentrated solution.
The level of the concentrated solution will rise or, if it is confined to an
enclosed space, its pressure will increase. The membrane separating the two
solutions is often called selectively permeable or semi-permeable because it
appears as if water molecules can pass through it more easily than sugar
molecules can.

Osmosis then is the passage of water across a selectively permeable membrane
from a dilute solution to a concentrated solution.

This is all you need to know in order to understand the effects of osmosis
in living organisms, But a more complete explanation is given below.


The current text book explanation for osmosis appears to have ignored the
effects of gravity on liquids. The constant pull of gravity acts differently
on concentrated solutions than dilute solutions i.e. The concentrated
solution is heavier than the dilute solution and will always initially
settle at the bottom of a reservoir or in this case a vessel.

Chapter 7 Transport in plants

page 71

The main force which draws water from the soil and through the plant is
caused by a process called transpiration. Water evaporates from the leaves
and causes a kind of 'suction ' which pulls water up the stem. The water
travels up the vessels in the vascular bundles and this flow of water is
called the transpiration stream. The water vapour passes by diffusion
through the air spaces in the mesophyll and out of the stomata. It is this
loss of water vapour from the leaves which is called transpiration. The cell
walls which are losing water in this way replace it by drawing water from
the nearest vein. Most of this water travels along the cell walls without
actually going inside the cells. Thousands of leaf cells are evaporating
water like this and drawing water to replace it from the xylem vessels in
the veins. As a result , water is pulled through the xylem vessels and up
the stem from the roots. This transpiration pull is strong enough to draw up
water 50 metres or more in trees.

Page 72

Most of this water evaporates from the leaves; only a tiny fraction is
retained for photosynthesis and to maintain the turgor of the cells. The
advantage to the plant of this excessive evaporation is not clear.

A rapid water flow may be needed to obtain sufficient mineral salts, which
are in very dilute solution in the soil. Evaporation may also help to cool
the leaf when exposed to intense sunlight.

Against the first possibility it has to be pointed out that, in some cases,
an increased transpiration rate does not increase the uptake of minerals.

Many biologists regard transpiration as an inevitable consequence of
photosynthesis, in order to photosynthesise, a leaf has to take in carbon
dioxide from the air. The pathway that lets carbon dioxide in will also let
water vapour out whether the plant needs to lose water or not. In all
probability, plants have to maintain a careful balance between the optimum
intake of carbon dioxide and a damaging loss of water.

Page 73

Humidity if the air is very humid, i.e. contains a great deal of water
vapour, it can accept very little more from the plants and so transpiration
slows down. In dry air, the diffusion of water vapour from the leaf to the
atmosphere will be rapid. ( " I will deal with this point later on because
it is very important and has implications for human health ") Air Movements:
In still air, the region round a transpiring leaf will become saturated with
water vapour so that no more can escape from the leaf. In these conditions,
transpiration slows down. In moving air the water vapour will be swept away
from the leaf as fast as it diffuses out. This will Speed up the
transpiration. Furthermore, when the sun shines on the leaves, they will
absorb heat as well as light. This warms them up and increases the rate of

Page 73 continued Water movement in the xylem

You may have learned in physics that you cannot draw water up by suction to
a height of more than about ten metres. Many trees are taller than this yet
they can draw up water effectively. The explanation offered is that, in long
vertical columns of water in very thin tubes, the attractive forces between
the water molecules are greater than the forces trying to separate them. So
in effect the transpiration stream is pulling up thin threads of water which
resist the tendency to break.

There are still problems however, it is likely that the water columns in
some of the vessels do have air breaks in them and yet the total water flow
is not affected. The evidence all points to the non-living xylem vessels as
the main route by which water passes from the soil to the leaves.

"This statement suggests that the long thin tubes of the tree ,are used for
water transport, which are none-living , therefore must represent the tubes
used in my experiments at Brixham."

Page 74

Root Pressure

In Experiment 8 on page 79 it is demonstrated that liquid may be forced up a
stem by root pressure from the root system. The usual explanation for this
is that the cell sap in the root hairs is more concentrated than the

soil water and so water enters by osmosis (see page 36). The water passes
from cell to cell by osmosis and is finally forced into the xylem vessels in
the centre of the root and up the stem.

This is rather an elaborate model from very little evidence. For example, a
gradient of falling osmotic potentials from the outside to the inside of a
root has not been demonstrated. However, there is some supporting evidence
for the movement of water as a result of root pressure.

root pressures of 1-2 atmospheres have been recorded, and these would
support columns of water 10 or 20 metres high. Some workers claim pressures
of up to eight atmospheres (i.e. 80 metres of water)

" A column of water 80 metres high would undoubtedly cause water pressures
of eight atmospheres at the roots. However It is very difficult to see how a
root could generate 8 atmospheres of pressure."

However, root pressure seems to occur mainly in the young herbaceous (i.e.
non-woody) plants or in woody plants early in the growing season and though
in many species it must contribute to water movements in the stem. The
observed rates of flow are too fast to be explained by root pressure alone.

Transport of salts

The liquid which travels in the xylem is not, in fact pure water. It is a
very dilute solution, containing from 0.1to1.0% dissolved solids, mostly
amino acids, other organic acids and mineral salts. The organic acids are
made in the roots; the mineral salts come from the soil. The faster the flow
in the transpiration stream, the more dilute is the xylem sap. Experimental
evidence suggests that salts are carried from the soil to the leaves mainly
in the xylem vessels.

Transport of food

The xylem sap is always a very dilute solution, but the Phloem sap may
contain up to 25 per cent of dissolved solids, The bulk of which consists of
sucrose and amino acids.

There is a good deal of evidence to support the view that sucrose amino
acids and may other substances are transported in the phloem. The movement
of water and salts in the xylem is always upwards, from the soil to the
leaf. But in the phloem the sap may be travelling up or down the stem. The
carbohydrates made in the leaf during photosynthesis are converted to
sucrose and carried out of the leaf to the stem. From here the sucrose may
pass upwards to growing buds and fruits or downwards to the roots and
storage organs. All parts of a plant which cannot photosynthesise will need
a supply of nutrients bought by the phloem. It is possible for substances to
be travelling upwards and downwards at the same time in the phloem.

"note the dual flow has been observed in experiments with concentrated
solution and water filled tubes."

Page 74 continued

There is no doubt that substances travel in the sieve tubes of the phloem
But the mechanism by which they are moved is not fully understood.

There are several theories, which attempt to explain how sucrose and other
solutes are transported in the phloem but none of them is entirely

Page 75

Uptake of water and salts

The water tension developed in the vessels by a rapidly transpiring plant is
thought to be sufficient to draw water through the root from the soil. The
precise pathway taken by the water is the subject of some debate, but the
path of least resistance seems to be in or between the cell walls rather
than through the cells.

When transpiration is slow, e.g. at night time or just before bud burst in a
deciduous tree, then osmosis may play a more important part in the uptake of

One problem for this explanation is that it has not been possible to
demonstrate that there is an osmotic gradient across the root cortex which
could produce this flow of water from cell to cell. Nevertheless, root
pressure developed probably by osmosis can be shown to force water up the
root system and into the stem

page 76

The methods by which roots take up salts from the soil are not fully
understood. Some salts may be carried in with the water drawn up by
transpiration and pass mainly along the cell walls in the root cortex and
into the xylem.

It may be that diffusion from a relatively high concentration in the soil to
a lower concentration in the root cells accounts for uptake of some
individual salts. But it has been shown (a) that salts can be taken from the
soil even when their concentration is below that in the roots and (b) that
anything which interferes with respiration impairs the uptake of salts. This
suggests that active transport (p.35) plays an important part in the uptake
of salts.

The thing that becomes clear from reading the established explanations for
water transport is that if it were a bucket, very little water would be
transported due to the large number of holes in it !

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