As to the Mode of Action of the Guard Cells in Dry Air
211
required to plasmolyze guard cells in epidermal samples taken at various
intervals. Unfortunately, sameples were allowed to equilibrate for 40min
with the sugar solutions, a period in which considerable solute exchange
may have taken place. Although StiHfelt, aware of the problem, argued that
the determinations would provide a relative indication of guard cell pressure
at the time the samples were taken, the evidence must be regarded as
suspect.
Meidner and Edwards (1975), also, had the idea that larger pressure in
the guard cells may be required to start the process of bending the ventral
walls than to continue the process. The context was a discussion of their
experiments (see also Meidner 1982) in which capillaries were inserted into
guard cells in intact leaves of Tradescantia virginiana and used to manipulate
turgor pressure. When a capillary was inserted into a guard cell of an open
stoma, turgor was lost and the pore closed partially, apparently in association
with penetration of cytoplasm into the capillary. On application of hydraulic
pressure in the capillary, the pore could be made to reopen up to, and
beyond its original width. However, it was impossible to open an initially
closed stoma by applying a pressure in one of its guard cells of 10 bar, the
maximum of which the apparatus was capable. An opening movement could
be achieved only with stomata in which the pore was already at least very
slightly open. The observations indicate, as SUUfelt implies, that stomata
may be capable of existing in either of at least two states, one closed and the
other partially open, at the one level of turgor pressure in the guard cells.
The contingency that this is due to an intrinsic mechanical property of
stomata and not to temporally varying elasticity of the guard cell wall (an
alternative suggestion by Meiduer and Edwards) is the keystone of the
hypothesis that will be advanced about stomatal responses to humidity.
10.3.2 Signals and Responses
Joseph Banks' observation that stomata close in dry air was noted by von
Mohl and contrasted with evidence that stomata were open in leaves free of
dew and in bright sun, conditions tending to promote rapid transpiration.
However, then, and for some time after, it was not fully understood that
stomata could respond to many different stimuli. There were those, such as
Schwendener, who held that light was the only stimulus and others, such as
Leitgeb (1886), who believed that change in leaf water status was the
predominant influence. The articles by Stahl and F. Darwin were perhaps
the first in which there emerged a more balanced concept of stomatal action.
With regard to the influence of atmospheric humidity, Darwin found that
stomata in plants transferred from a moist to a drier atmosphere closed
"without there being the slightest appearance of flaccidity in the leaves",
having taken pains to ensure that his observations were not confounded by
change in light intensity. However, Darwin had difficulty in understanding
211
required to plasmolyze guard cells in epidermal samples taken at various
intervals. Unfortunately, sameples were allowed to equilibrate for 40min
with the sugar solutions, a period in which considerable solute exchange
may have taken place. Although StiHfelt, aware of the problem, argued that
the determinations would provide a relative indication of guard cell pressure
at the time the samples were taken, the evidence must be regarded as
suspect.
Meidner and Edwards (1975), also, had the idea that larger pressure in
the guard cells may be required to start the process of bending the ventral
walls than to continue the process. The context was a discussion of their
experiments (see also Meidner 1982) in which capillaries were inserted into
guard cells in intact leaves of Tradescantia virginiana and used to manipulate
turgor pressure. When a capillary was inserted into a guard cell of an open
stoma, turgor was lost and the pore closed partially, apparently in association
with penetration of cytoplasm into the capillary. On application of hydraulic
pressure in the capillary, the pore could be made to reopen up to, and
beyond its original width. However, it was impossible to open an initially
closed stoma by applying a pressure in one of its guard cells of 10 bar, the
maximum of which the apparatus was capable. An opening movement could
be achieved only with stomata in which the pore was already at least very
slightly open. The observations indicate, as SUUfelt implies, that stomata
may be capable of existing in either of at least two states, one closed and the
other partially open, at the one level of turgor pressure in the guard cells.
The contingency that this is due to an intrinsic mechanical property of
stomata and not to temporally varying elasticity of the guard cell wall (an
alternative suggestion by Meiduer and Edwards) is the keystone of the
hypothesis that will be advanced about stomatal responses to humidity.
10.3.2 Signals and Responses
Joseph Banks' observation that stomata close in dry air was noted by von
Mohl and contrasted with evidence that stomata were open in leaves free of
dew and in bright sun, conditions tending to promote rapid transpiration.
However, then, and for some time after, it was not fully understood that
stomata could respond to many different stimuli. There were those, such as
Schwendener, who held that light was the only stimulus and others, such as
Leitgeb (1886), who believed that change in leaf water status was the
predominant influence. The articles by Stahl and F. Darwin were perhaps
the first in which there emerged a more balanced concept of stomatal action.
With regard to the influence of atmospheric humidity, Darwin found that
stomata in plants transferred from a moist to a drier atmosphere closed
"without there being the slightest appearance of flaccidity in the leaves",
having taken pains to ensure that his observations were not confounded by
change in light intensity. However, Darwin had difficulty in understanding