210
I.R. Cowan
walls, has shortened. Thirdly, something the reader may wish to verify by
measurement, the perimeter of the pore, defined in the same way, has
changed very little, if at all. In most descriptions and models of movement
in stomata having kidney-shaped guard cells (Raschke 1979; 'Sharpe et al.
1987), the major axis of the pore is constant in -length and opening of the
pore results from a stretching of the ventral walls. That the walls are
stretched in widely open stomata is undoubtedly true, and it is perhaps
placing an unreasonable burden on Schwendener's drawing to conclude that
they are not in the early stages of opening also. However, the drawing does
serve to emphasize that what appears to be a rather insignificant diminution
of length is sufficient to accommodate a visually much more impressive
increase in width of the pore without stretching of the walls.
Most investigations of the way in which stomatal aperture, guard cell
volume, and guard cell turgor are interrelated have used epidermal strips
exposed to solutions of varying osmotic pressure. All of them, including
those with epidermal strips in which the subsidiary cells as well as guard cells
have been maintained intact (e.g., see MacRobbie 1987), have yielded
results which have been interpreted as a monotonic increase of aperture and
volume with pressure. (I use the word interpreted, because sometimes the
number of data points has been inadequate to demonstrate any conflict
there may have been with what has been a common assumption.) There
have, however, been at least two suggestions that the state of affairs may
sometimes be more complex.
Stalfelt (1927), on the basis of observations of the time course of widths
of the stomatal apparatus as a whole and of the pore in Vicia faba following
illumination, distinguished two phases of stomatal movement. In the initial
Spanniungsphase, the length of which depends on the intensity of illumination, the guard cells thicken but the pore does not open. Just before the
pore is observed to open, the cells narrow somewhat. Once opening is under
way the guard cells thicken again, and the width of the whole apparatus
increases in concert with increase in the width of the pore during what
Stalfelt called the motorische Phase. Stalfelt explained the events he saw in
terms of pressure in the guard cell thus:
"Es geniigt hierbei nicht, daB der Turgor den Druckwert der Nebenzellen erreicht.
Erst wenn der Druck der SchlieBzellen einen gewissen Mehrbetrag erreicht hat,
setzt die Bewegung ein. Sie kann dann in einer Zeit von viel geringeren Drucken
weitergetrieben werden, d.h. die Bewegung muB sich unter vermindertem Drucke
fortsetzen. ,,2
Some support for the idea that guard cell pressure decreases at the onset of
the motorische Phase was provided by determinations of sugar concentrations
21t is not sufficient that the magnitude of the turgor pressure of the subsidiary cells be
attained. The movement first begins when the pressure of the guard cells exceeds (that of
the subsidiary cells) by a certain amount. Then the cells can be driven apart during a
period of much smaller pressure, i.e., the movement continues under reduced pressure.
I.R. Cowan
walls, has shortened. Thirdly, something the reader may wish to verify by
measurement, the perimeter of the pore, defined in the same way, has
changed very little, if at all. In most descriptions and models of movement
in stomata having kidney-shaped guard cells (Raschke 1979; 'Sharpe et al.
1987), the major axis of the pore is constant in -length and opening of the
pore results from a stretching of the ventral walls. That the walls are
stretched in widely open stomata is undoubtedly true, and it is perhaps
placing an unreasonable burden on Schwendener's drawing to conclude that
they are not in the early stages of opening also. However, the drawing does
serve to emphasize that what appears to be a rather insignificant diminution
of length is sufficient to accommodate a visually much more impressive
increase in width of the pore without stretching of the walls.
Most investigations of the way in which stomatal aperture, guard cell
volume, and guard cell turgor are interrelated have used epidermal strips
exposed to solutions of varying osmotic pressure. All of them, including
those with epidermal strips in which the subsidiary cells as well as guard cells
have been maintained intact (e.g., see MacRobbie 1987), have yielded
results which have been interpreted as a monotonic increase of aperture and
volume with pressure. (I use the word interpreted, because sometimes the
number of data points has been inadequate to demonstrate any conflict
there may have been with what has been a common assumption.) There
have, however, been at least two suggestions that the state of affairs may
sometimes be more complex.
Stalfelt (1927), on the basis of observations of the time course of widths
of the stomatal apparatus as a whole and of the pore in Vicia faba following
illumination, distinguished two phases of stomatal movement. In the initial
Spanniungsphase, the length of which depends on the intensity of illumination, the guard cells thicken but the pore does not open. Just before the
pore is observed to open, the cells narrow somewhat. Once opening is under
way the guard cells thicken again, and the width of the whole apparatus
increases in concert with increase in the width of the pore during what
Stalfelt called the motorische Phase. Stalfelt explained the events he saw in
terms of pressure in the guard cell thus:
"Es geniigt hierbei nicht, daB der Turgor den Druckwert der Nebenzellen erreicht.
Erst wenn der Druck der SchlieBzellen einen gewissen Mehrbetrag erreicht hat,
setzt die Bewegung ein. Sie kann dann in einer Zeit von viel geringeren Drucken
weitergetrieben werden, d.h. die Bewegung muB sich unter vermindertem Drucke
fortsetzen. ,,2
Some support for the idea that guard cell pressure decreases at the onset of
the motorische Phase was provided by determinations of sugar concentrations
21t is not sufficient that the magnitude of the turgor pressure of the subsidiary cells be
attained. The movement first begins when the pressure of the guard cells exceeds (that of
the subsidiary cells) by a certain amount. Then the cells can be driven apart during a
period of much smaller pressure, i.e., the movement continues under reduced pressure.