208
I.R. Cowan
as the direct humidity response. Actually, the term direct is not entirely
apposite either, as will be understood later, but the reason for using it was
well put by Schulze et al. "When the stomata close at low air humidity the
water content of the leaves increases. The stomata open again at high air
humidity in spite of a decrease in leaf water content. This excludes a
reaction via the water potential in the leaf tissue and proves that the
stomatal aperture has a direct response to the evaporative conditions in
the atmosphere". Schulze et al. implied that the mechanism might be asociated with peristomatal transpiration; that is to say, loss of water from the
external surface of the leaf in the neighborhood of the stomatal apparatus
taking place at a rate sufficient to affect the turgor of the guard cells.
Raschke (1970) had been drawn to the same conclusion by observations
made with detached leaves of Zea mays.
Numerous experiments have now confirmed the existence of the direct
humidity response (Schulze and Hall 1982). The argument of Schulze et al.
relating to mechanism seemed to be compelling (Cowan 1977; Farquhar
1978). It conflicted with the view, widely held at that time, that the response
of stomata to ambient evaporative conditions is indirect, a feedback process
depending on the leaf-internal consequences of variation in rate of transpiration. However, I shall try to show that what had been observed in
Wiirzburg and in the Negev is not inconsistent with the feedback hypothesis;
that the so-called direct humidity response could be brought about indirectly
and that the explanation might rest with a generally unsuspected mechanical
property of guard cells. Before doing so, I shall draw attention to some
relevant studies of stomata that preceded the two that have been described,
and some that followed.
10.3 Some Relevant Observations
10.3.1 On Stomatal Mechanics
It was von Mohl (1856) who, having made observations of stomatal movements in separated epidermis bathed in sugar solution of varied concentration, concluded that increase in guard cell volume and pore aperture
resulted from increase in hydraulic pressure in the guard cell. Von Mohl also
found by puncturing guard cells and subsidiary cells that the turgor pressure
of the latter tended to diminish stomatal aperture. He described the interaction of guard cells and epidermal cells as one of "antagonism". A wellknown drawing (Fig. 10.2) by Schwendener (1881) serves to illustrate
the anatomy and change in anatomy associated with these movements in
stomata having "kidney-shaped" guard cells, i.e., stomata in most dicotyledonous plants. Increase in pressure in the guard cell, so it is thought,
causes the relatively thin dorsal walls to stretch and protrude into the
I.R. Cowan
as the direct humidity response. Actually, the term direct is not entirely
apposite either, as will be understood later, but the reason for using it was
well put by Schulze et al. "When the stomata close at low air humidity the
water content of the leaves increases. The stomata open again at high air
humidity in spite of a decrease in leaf water content. This excludes a
reaction via the water potential in the leaf tissue and proves that the
stomatal aperture has a direct response to the evaporative conditions in
the atmosphere". Schulze et al. implied that the mechanism might be asociated with peristomatal transpiration; that is to say, loss of water from the
external surface of the leaf in the neighborhood of the stomatal apparatus
taking place at a rate sufficient to affect the turgor of the guard cells.
Raschke (1970) had been drawn to the same conclusion by observations
made with detached leaves of Zea mays.
Numerous experiments have now confirmed the existence of the direct
humidity response (Schulze and Hall 1982). The argument of Schulze et al.
relating to mechanism seemed to be compelling (Cowan 1977; Farquhar
1978). It conflicted with the view, widely held at that time, that the response
of stomata to ambient evaporative conditions is indirect, a feedback process
depending on the leaf-internal consequences of variation in rate of transpiration. However, I shall try to show that what had been observed in
Wiirzburg and in the Negev is not inconsistent with the feedback hypothesis;
that the so-called direct humidity response could be brought about indirectly
and that the explanation might rest with a generally unsuspected mechanical
property of guard cells. Before doing so, I shall draw attention to some
relevant studies of stomata that preceded the two that have been described,
and some that followed.
10.3 Some Relevant Observations
10.3.1 On Stomatal Mechanics
It was von Mohl (1856) who, having made observations of stomatal movements in separated epidermis bathed in sugar solution of varied concentration, concluded that increase in guard cell volume and pore aperture
resulted from increase in hydraulic pressure in the guard cell. Von Mohl also
found by puncturing guard cells and subsidiary cells that the turgor pressure
of the latter tended to diminish stomatal aperture. He described the interaction of guard cells and epidermal cells as one of "antagonism". A wellknown drawing (Fig. 10.2) by Schwendener (1881) serves to illustrate
the anatomy and change in anatomy associated with these movements in
stomata having "kidney-shaped" guard cells, i.e., stomata in most dicotyledonous plants. Increase in pressure in the guard cell, so it is thought,
causes the relatively thin dorsal walls to stretch and protrude into the