226
I.R. Cowan
tension in the radiating micro fibrils to decrease and the net forces on
the walls B-B to be directed inwards. Even so, the system would remain
unstable in the absence of continuous adjustment of pressure to maintain
the balance between the forces parallel to, and normal to the axis of the
pore. Figure 10.8 shows an example of variations of pore area and guard cell
volume with balancing pressure, estimated with particular assumed elastic
properties relating to bending at B and wall extension at S, and with P e
taken as constant. Increment in guard cell area (equivalent to cell volume
in a two-dimensional representation of stomatal movement) has been calculated on the basis that the increment of cell + pore area is m x (length of
closed pore) x (width of pore).
What has been described here is a grossly simplified model of stomatal
action. Nevertheless it serves to demonstrate that it is mechanically possible
for an increase in stomatal aperture and guard cell volume to be associated
with decrease in the pressure in the guard cells. The relationships in Fig.
10.8 have the characteristics I have postulated in this attempt to account for
the direct humidity response as a manifestation of an indirect, feedback
process.
10.5 Conclusions
Three ideas have been advanced. The most general of them is that the direct
humidity response is due to a negative feedback system with a rather special
property: the feedback gain sometimes exceeds unity and yet the system
remains stable because it contains an element, a subsidiary, positive feedback loop which, in isolation, is unstable. This, if correct, is important, for it
suggests that the response is susceptible to a variety of explanations of a
kind not previously considered. It could hardly fail to have implications
for other aspects of stomatal function, such as the response to light. The
possibility that the system as a whole sometimes becomes unstable, control
then having an "on-off"characteristic, might be the basis of heterogeneity
and patchiness of stomatal aperture.
The second idea is to do with the specific nature of the internal, intrinsically unstable element. That the instability is due to decrease in pressure
with increase in volume in the guard cell, perhaps allied to a tendency for
osmotic pressure to be maintained more nearly constant than it would be if
solute were conserved, is not the only suggestion possible. However, it
seems more plausible than any other I have been able to devise. There is at
least some slight support for it in the work of Stalfelt, and Meidner and
Edwards.
Third is the idea that decrease in pressure in the guard cell with increase
in volume and stomatal aperture is associated with increase in a bending
I.R. Cowan
tension in the radiating micro fibrils to decrease and the net forces on
the walls B-B to be directed inwards. Even so, the system would remain
unstable in the absence of continuous adjustment of pressure to maintain
the balance between the forces parallel to, and normal to the axis of the
pore. Figure 10.8 shows an example of variations of pore area and guard cell
volume with balancing pressure, estimated with particular assumed elastic
properties relating to bending at B and wall extension at S, and with P e
taken as constant. Increment in guard cell area (equivalent to cell volume
in a two-dimensional representation of stomatal movement) has been calculated on the basis that the increment of cell + pore area is m x (length of
closed pore) x (width of pore).
What has been described here is a grossly simplified model of stomatal
action. Nevertheless it serves to demonstrate that it is mechanically possible
for an increase in stomatal aperture and guard cell volume to be associated
with decrease in the pressure in the guard cells. The relationships in Fig.
10.8 have the characteristics I have postulated in this attempt to account for
the direct humidity response as a manifestation of an indirect, feedback
process.
10.5 Conclusions
Three ideas have been advanced. The most general of them is that the direct
humidity response is due to a negative feedback system with a rather special
property: the feedback gain sometimes exceeds unity and yet the system
remains stable because it contains an element, a subsidiary, positive feedback loop which, in isolation, is unstable. This, if correct, is important, for it
suggests that the response is susceptible to a variety of explanations of a
kind not previously considered. It could hardly fail to have implications
for other aspects of stomatal function, such as the response to light. The
possibility that the system as a whole sometimes becomes unstable, control
then having an "on-off"characteristic, might be the basis of heterogeneity
and patchiness of stomatal aperture.
The second idea is to do with the specific nature of the internal, intrinsically unstable element. That the instability is due to decrease in pressure
with increase in volume in the guard cell, perhaps allied to a tendency for
osmotic pressure to be maintained more nearly constant than it would be if
solute were conserved, is not the only suggestion possible. However, it
seems more plausible than any other I have been able to devise. There is at
least some slight support for it in the work of Stalfelt, and Meidner and
Edwards.
Third is the idea that decrease in pressure in the guard cell with increase
in volume and stomatal aperture is associated with increase in a bending