Although chloroplasts
are found in the cells of young stems and immature fruits, leaves are the real photosynthetic
factories of the plant.
A cross section through the
blade of a typical leaf reveals 4 distinct tissue layers.
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Stomata under microscope taken from http://www.microscopy-uk.org.uk/mag/articles/stomata.html |
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Diagram of stomata taken from http://www.puc.edu/Faculty/Gilbert_Muth/art0077.jpg |
Not only must the cells of the
palisade and spongy layers be close to their air supply
but they must be close to a
leaf vein with its
The photo shows the network of leaf veins in
a maple leaf. Probably no cell in the spongy layer is more than two cells away
from a vein.
The xylem and phloem of veins is often
surrounded by layers of sclerenchyma
cells. These impart strength to the vein providing a stiff framework to support
the soft tissues of the leaf blade.
In order to carry on photosynthesis,
green plants need a supply of carbon dioxide and a means of disposing of
oxygen. In order to carry on cellular
respiration, plant cells need oxygen and a means of disposing of carbon dioxide (just as animal
cells do).
Unlike animals, plants have
no specialized organs for gas exchange (with the few inevitable exceptions!).
The are several reasons they can get along without them:
The exchange of oxygen and carbon dioxide in
the leaf (as well as the loss of water vapor in transpiration)
occurs through pores called stomata (singular = stoma).
Normally stomata open when
the light strikes the leaf in the morning and close during the night.
The immediate cause is a change in the turgor
of the guard cells. The inner wall of each guard cell is thick and
elastic. When turgor develops within the two guard cells flanking each stoma,
the thin outer walls bulge out and force the inner walls into a crescent shape.
This opens the stoma. When the guard cells lose turgor, the elastic inner walls
regain their original shape and the stoma closes.
Time |
Osmotic Pressure, lb/in2 |
7 A.M. |
212 |
11 A.M. |
456 |
5 P.M. |
272 |
12 midnight |
191 |
The table shows the osmotic pressure
measured at different times of day in typical guard cells. The osmotic pressure
within the other cells of the lower epidermis remained constant at 150 lb/in2.
When the osmotic pressure of the guard cells became greater than that of the
surrounding cells, the stomata opened. In the evening, when the osmotic
pressure of the guard cells dropped to nearly that of the surrounding cells,
the stomata closed.
The increase in osmotic
pressure in the guard cells is caused by an uptake of potassium ions (K+).
The concentration of K+ in open
guard cells far exceeds that
in the surrounding cells. This is how it accumulates:
Although open stomata are essential for
photosynthesis, they also expose the plant to the risk of losing water through transpiration.
Some 90% of the water taken up by a plant is lost in transpiration.
Abscisic
acid (ABA) is the hormone that triggers closing of the stomata when soil
water is insufficient to keep up with transpiration (which often occurs around
mid-day).
The mechanism:
The density of stomata on a
leaf varies with such factors as:
These data can be quantified by determining
the stomatal index: the ratio of the number of stomata in a given area
divided by the total number of stomata and other epidermal cells in that same
area.
How does the plant determine how many
stomata to produce?
It turns out that the mature leaves on the
plant detect the conditions around them and send a signal (its nature still
unknown) that adjusts the number of stomata that will form on the developing
leaves.
Two experiments (reported by Lake et al., in
Nature, 411:154, 10 May 2001):
Because CO2
levels and stomatal index are inversely related, could fossil leaves tell us
about past levels of CO2 in the atmosphere? Yes. As reported by
Gregory Retallack (in Nature, 411:287, 17 May 2001), his study of
the fossil leaves of the ginkgo
and its relatives shows:
Both these periods are known
from geological evidence to have been times of
These studies also lend
support to the importance of carbon dioxide as a greenhouse gas playing an
important role in global
warming.
Woody stems and mature roots are sheathed in
layers of dead cork cells impregnated with suberin — a waxy, waterproof
(and airproof) substance. So cork is as impervious to oxygen and carbon dioxide
as it is to water.
However, the cork of both
mature roots and woody stems is perforated by nonsuberized pores called lenticels.
These enable oxygen to reach the intercellular spaces of the interior tissues
and carbon dioxide to be released to the atmosphere.
The photo shows the lenticels in the bark of
a young stem.
In many annual plants, the stems are green
and almost as important for photosynthesis as the leaves. These stems use
stomata rather than lenticels for gas exchange.
Transpiration is the evaporation of water from
plants. It occurs chiefly at the leaves while their stomata
are open for the passage of CO2 and O2 during photosynthesis.
But air that is not fully saturated with
water vapor (100% relative humidity) will dry the surfaces of cells with which
it comes in contact. So the photosynthesizing leaf loses substantial amount of
water by evaporation. This transpired water must be replaced by the transport
of more water from the soil to the leaves through the xylem
of the roots and stem.
Transpiration is not simply
a hazard of plant life. It is the "engine" that pulls water up from
the roots to:
1. Light
Plants transpire more rapidly in the light than in the dark. This is largely
because light stimulates the opening of the stomata (mechanism).
Light also speeds up transpiration by warming the leaf.
2. Temperature
Plants transpire more rapidly at higher temperatures because water evaporates
more rapidly as the temperature rises. At 30°C, a leaf may transpire three
times as fast as it does at 20°C.
3. Humidity
The rate of diffusion
of any substance increases as the difference in concentration of the substances
in the two regions increases. When the surrounding air is dry, diffusion of
water out of the leaf goes on more rapidly.
4. Wind
When there is no breeze, the air surrounding a leaf becomes increasingly humid
thus reducing the rate of transpiration. When a breeze is present, the humid
air is carried away and replaced by drier air.
5. Soil water
A plant cannot continue to transpire rapidly if its water loss is not made up
by replacement from the soil. When absorption of water by the roots fails to
keep up with the rate of transpiration, loss of turgor
occurs, and the stomata close. This immediately reduces the rate of
transpiration (as well as of photosynthesis). If the loss of turgor extends to
the rest of the leaf and stem, the plant wilts.
The volume of water lost in transpiration
can be very high. It has been estimated that over the growing season, one acre
of corn plants may transpire 400,000 gallons of water. As liquid water, this
would cover the field with a lake 15 inches deep. An acre of forest probably
does even better.
Most plants secure the water and minerals
they need from their roots.
The path taken is: soil -> roots ->
stems -> leaves
The minerals (e.g., K+, Ca2+)
travel dissolved in the water (often accompanied by various organic molecules
supplied by root cells).
Less than 1% of the water reaching the
leaves is used in photosynthesis and plant growth. Most of it is lost in
transpiration.
However, transpiration does serve two useful
functions:
Water and minerals enter the root by
separate paths which eventually converge in the stele.
Soil water enters the root through its
epidermis. It appears that water then travels in both
However, the inner boundary of the cortex,
the endodermis, is impervious to water because of a band of suberized
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.
Once inside the stele, water is again free
to move between cells as well as through them. In young roots, water enters
directly into the xylem vessels and/or tracheids. These are nonliving
conduits so are part of the apoplast.
Once in the xylem, water with the minerals that
have been deposited in it (as well as occasional organic molecules supplied by
the root tissue) move up in the vessels and tracheids.
At any level, the water can leave the xylem
and pass laterally to supply the needs of other tissues.
At the leaves, the xylem passes into the
petiole and then into the veins of the leaf. Water leaves the finest veins and
enters the cells of the spongy
and palisade layers. Here some of the water may be used in metabolism, but
most is lost in transpiration.
Minerals enter the root by active
transport into the symplast of epidermal cells and move toward and into the
stele through the plasmodesmata connecting the cells.
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.
Observations
In 1895, the Irish plant physiologists H. H.
Dixon and J. Joly proposed that water is pulled up the plant by tension
(negative pressure) from above.
As we have seen, water is continually being
lost from leaves by transpiration. Dixon and Joly believed that the loss of
water in the leaves exerts a pull on the water in the xylem ducts and draws
more water into the leaf.
But even the best vacuum pump can pull water
up to a height of only 34 ft or so. This is because a column of water that high
exerts a pressure (~15 lb/in2) just counterbalanced by the pressure
of the atmosphere. How can water be drawn to the top of a sequoia (the tallest
is 370 feet high)? Taking all factors into account, a pull of at least 270
lb/in2 is probably needed.
The answer to the dilemma lies the cohesion
of water molecules; that is the property of water molecules to cling to each
through the hydrogen bonds they form.
When water is confined to tubes of very
small bore, the force of cohesion between water molecules imparts great
strength to the column of water. Tensions as great as 3000 lb/in2
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.
Because of the critical role of cohesion,
the transpiration-pull theory is also called the cohesion theory.
The graph shows the
results of obtained by D. T. MacDougall when he made continuous measurements of
the diameter of a Monterey pine. The diameter fluctuated on a daily basis
reaching its minimum when the rate of transpiration reached its maximum
(around noon)
If forced to take
water from a sealed container, the vine does so without any decrease in rate,
even though the resulting vacuum becomes so great that the remaining water
begins to boil spontaneously. (The boiling temperature of water decreases as
the air pressure over the water decreases, which is why it takes longer to boil
an egg in Denver than in New Orleans.)
When water is placed under a high vacuum,
any dissolved gases come out of solution as bubbles (as we saw above with the
rattan vine). This is called cavitation. Any impurities in the water
enhance the process. So measurements showing the high tensile strength of water
in capillaries require water of high purity — not the case for sap in the
xylem.
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.
By spinning branches in a centrifuge, it has
been shown that water in the xylem avoids cavitation at negative pressures
exceeding 225 lb/in2. And the fact that sequoias can successfully
lift water 358 ft (109 m) — which would require a tension of 270 lb/in2
— indicates that cavitation is avoided even at that value.
However, such heights may be
approaching the limit for xylem transport. (The tallest tree ever measured, a
Douglas fir, was 413 ft. high.) Measurements close to the top of the tallest
living sequoia (370 ft high) show that the high tensions needed to get water up
there have resulted in:
(See Koch, G. W. et al.,
Nature, 22 April 2004.)
When a tomato plant is
carefully severed close to the base of the stem, sap oozes from the stump. The
fluid comes out under pressure which is called root pressure.
Root pressure is created by the osmotic
pressure of xylem sap which is, in turn, created by dissolved
that have been actively
transported into the apoplast of the stele.
One important example is the sugar maple
when, in very early spring, it hydrolyzes
the starches stored in its roots into sugar. This causes water to pass by
osmosis through the endodermis and into the xylem ducts. The continuous inflow
forces the sap up the ducts.
Although root pressure plays a role in the
transport of water in the xylem in some plants and in some seasons, it does not
account for most water transport.
So although root pressure may play a
significant role in water transport in certain species (e.g., the coconut palm)
or at certain times, most plants meet their needs by transpiration-pull.
Adapted from Kimball's Biology Pages: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages