As to the Mode of Action of the Guard Cells in Dry Air
219
change in guard cell volume while the stomata were first caused to close and
later to open. If adjustment took place to the extent that n returned to
the same value after each disturbance, then the shape of the relationship
between V and P that we seek would differ from that between g and V only
insofar as the function g(V) may be curvilinear. If, at the other extreme, no
adjustment took place at all so that II varied inversely with volume, then
dP
d'P dg
II
- = - . - - -
dV dg dV V'
from which it is seen that dP/dV is negative over a range of g encompassing
and extending beyond that in which d'P Idg is negative. Nevertheless, it may
be assumed there is a limit to this trend at large volume, the forces resisting
further distension of a greatly swollen guard cell becoming sufficiently large
for dP/dV to be positive. It appears that the V(P) relationship must have the
same essential features of shape as the g('P) relationship in Fig. lO.4a.
If guard cell expansion has the characteristics described, it poses novel
questions about the function and structure of the stomatal apparatus.
10.4.2 Of Bubbles and Balloons
Reproduced in Fig. 10.5 is the volume:pressure relationship of a rubber
balloon (Bini Balloons, made in Denmark). I had been given to think of its
relevance as an analogy to the functioning of guard cells 15 years ago
(Cowan 1977) when I wrote of the observations of Meidner and Edwards
using a pressure probe to manipulate guard-cell turgor that they, " ... might
be thought to indicate that the initial expansion of the guard cell is essentially an irreversible process - irreversible in the thermodynamic sense, that
is - the internal pressure decreasing with increase in volume as with the
inflation of a rubber balloon". I further envisaged, in the same article, that
decline of conductance with increase of water potential in guard cell might
have something to do with the experiments of Schulze et al. However, I
rejected the idea on the grounds that the stomatal control system would
then be unstable. That was a mistake: stomata may sometimes be unstable,
and the dynamics of balloons sometimes are not.
Figure 10.5 quantifies a matter of common practical experience. There is
a Spannungsphase in which one needs to exert a greater pressure to initiate
inflation than to continue inflation once a certain volume has been achieved.
It is succeeded by a motorische Phase, increase in volume being accompanied by decline in pressure. Eventually, at large volume, the balloon
reverts to a "normal" characteristic, further increase in volume requiring
increase in pressure up to, and beyond the initial, threshold pressure.
Remarkably, the characteristic of the balloon in the motorische Phase is
somewhat like that of a soap bubble; pressure varies approximately as the
219
change in guard cell volume while the stomata were first caused to close and
later to open. If adjustment took place to the extent that n returned to
the same value after each disturbance, then the shape of the relationship
between V and P that we seek would differ from that between g and V only
insofar as the function g(V) may be curvilinear. If, at the other extreme, no
adjustment took place at all so that II varied inversely with volume, then
dP
d'P dg
II
- = - . - - -
dV dg dV V'
from which it is seen that dP/dV is negative over a range of g encompassing
and extending beyond that in which d'P Idg is negative. Nevertheless, it may
be assumed there is a limit to this trend at large volume, the forces resisting
further distension of a greatly swollen guard cell becoming sufficiently large
for dP/dV to be positive. It appears that the V(P) relationship must have the
same essential features of shape as the g('P) relationship in Fig. lO.4a.
If guard cell expansion has the characteristics described, it poses novel
questions about the function and structure of the stomatal apparatus.
10.4.2 Of Bubbles and Balloons
Reproduced in Fig. 10.5 is the volume:pressure relationship of a rubber
balloon (Bini Balloons, made in Denmark). I had been given to think of its
relevance as an analogy to the functioning of guard cells 15 years ago
(Cowan 1977) when I wrote of the observations of Meidner and Edwards
using a pressure probe to manipulate guard-cell turgor that they, " ... might
be thought to indicate that the initial expansion of the guard cell is essentially an irreversible process - irreversible in the thermodynamic sense, that
is - the internal pressure decreasing with increase in volume as with the
inflation of a rubber balloon". I further envisaged, in the same article, that
decline of conductance with increase of water potential in guard cell might
have something to do with the experiments of Schulze et al. However, I
rejected the idea on the grounds that the stomatal control system would
then be unstable. That was a mistake: stomata may sometimes be unstable,
and the dynamics of balloons sometimes are not.
Figure 10.5 quantifies a matter of common practical experience. There is
a Spannungsphase in which one needs to exert a greater pressure to initiate
inflation than to continue inflation once a certain volume has been achieved.
It is succeeded by a motorische Phase, increase in volume being accompanied by decline in pressure. Eventually, at large volume, the balloon
reverts to a "normal" characteristic, further increase in volume requiring
increase in pressure up to, and beyond the initial, threshold pressure.
Remarkably, the characteristic of the balloon in the motorische Phase is
somewhat like that of a soap bubble; pressure varies approximately as the