Agriculture Reference
In-Depth Information
although some will also pass by osmosis from cell to
cell too (see p. 120).
The central region, the
stele
, is separated from the
cortex by a single layer of cells, the
endodermis
,
which has the function of controlling the passage
of water and minerals into the stele. A waxy strip
forming part of the cell wall of the endodermal cells
(the Casparian strip) prevents water from moving
between the cells and all the water now has to pass
across the endodermal cell membranes and into
the cells by osmosis. The cell membrane also acts
as a control point for mineral uptake as only certain
minerals are able to cross it. Water passes through
the endodermis and pericycle to the
xylem
tissue
(see p. 79), which transports the water and dissolved
mineral nutrients up the stem to the leaves.
Root anatomy is described in Chapter 6.
Guard cells
Stomata
Figure 10.5
Scanning electron microscope image
showing stomata on the surface of a
Betula pendula
(birch) leaf. Each small pore (stoma) is surrounded by a
pair of guard cells (source: V. Vaslap and T. Jarveots)
Movement of water up the stem
Water is 'sucked' up the
xylem tissue
of the stem
(Figure 10.3) by a process called
transpiration
pull
, that is, as water is lost from the leaves it is
replaced by water which is drawn up the stem
carrying dissolved minerals with it (see below). The
evaporation of water from the cells of the leaf means
that in order for the leaf to remain turgid, which is
important for efficient photosynthesis, the water lost
must be replaced by water in the xylem. Pressure is
created in the xylem and water moves up through the
stem and leaf petiole by suction as long as the water
forms a continuous column. If the water column in the
xylem is broken, for example when a stem of a flower
is cut, air moves into the xylem and may restrict the
further movement of water when the cut flower is
placed in a vase of water, much like an air bubble in a
straw. However, by cutting the stem under water the
column is maintained and water continues to enter
and pass up the plant.
Stem anatomy is described in Chapter 6.
diffuses out of the leaf into the surrounding air (see
Figure 9.5) through the
stomatal pores
because
there is a lower relative humidity in the surrounding
air compared with inside the leaf. The loss of water
vapour from the leaf is called
transpiration
.
Transpiration
is the evaporation of water vapour
from the leaves and other plant surfaces.
Transpiration
Any plant takes up a lot of water through its roots -
for example, a tree can transport about 1,000 litres
(about 200 gallons) a day. Approximately 98% of the
water taken up moves through the plant and is lost by
transpiration; only about 2% is retained as part of the
plant's structure, and a yet smaller amount is used
up in photosynthesis. The seemingly extravagant loss
through leaves is due to the unavoidably large pores
in the leaf surface (
stomata
), essential for carbon
dioxide uptake for photosynthesis. Stomata (singular:
stoma) each consist of a central opening or pore
surrounded by two sausage-shaped guard cells which
control their opening. When the guard cells are fully
turgid the stomata remain open whereas loss of turgor
in the guard cells causes them to close to reduce
water loss (Figure 10.5).
A remarkable aspect of transpiration is that water can
be pulled ('sucked') such a long way to the tops of
tall trees. Engineers have long known that columns
of water break when they are more than about 10
m long, and yet tall trees such as
Sequoiadendrom
Xylem tissue
transports water and dissolved
mineral nutrients from the roots, up the stem to
the leaves and other plant organs
.
10
Movement of water in the leaf
On reaching the leaf, water is distributed in the fine
network of veins and passes out of the xylem (Figure
10.3). It flows between the leaf cells and also passes
from cell to cell by osmosis as in the root. Eventually
it evaporates from the cell surfaces into the air spaces
of the leaf mesophyll. From here, the water vapour
