10 As to the Mode of Action of the Guard Cells
in Dry Air
I.R. Cowan
10.1 Introduction
The first observation that stomata tend to close in dry air seems to have
been made by one whose name is not usually found in reviews of stomatal
physiology. Joseph Banks (1805) wrote that the pores in the stems of wheat,
"which exist also on the leaves and glumes", are shut in dry weather and
open in wet. The context is an essay on the cause of mildew in wheat, in
which Banks supposes that the infection takes place through the stomata.
He presumed the pores are "a provision intended no doubt to compensate,
in some measure, the want of locomotion in vegetables" and remarked that
"A plant cannot when thirsty go to the brook and drink: but it can open
innumerable orifices for the reception of every degree of moisture, which
either falls in the shape of rain or dew, or is separated from the mass of
water always held in solution in the atmosphere .... "
Nearly a century later, Francis Darwin (1898) speculated on the role of
stomatal response to humidity in the following passage, from which the title
of this article is taken. "Some interesting questions arise as to the mode of
action of the guard cells in dry air. Can we look at guard cells as senseorgans which, when the leaf is threatened by want of water, perceive the
danger before the rest of the leaf? This idea is not wholly fanciful. Stahl has
given some reason to believe that the transpiration of the guard cells is
especially active; in dry air, or with a diminished water-supply, they should
therefore be the first to suffer." In the work to which Darwin refers, Stahl
(1894) suggested that the most important step in the "Emancipation" of
plants from high atmospheric humidity lay in the capability of stomata to
close. Ernst Stahl, who qualified as lecturer in Wiirzburg before moving to
Jena, was an accomplished ecophysiologist with an ingenuity that will be
indicated later.
The idea, then, that stomatal response to atmospheric conditions promoting transpiration is adapted to avert impending danger is quite an old one.
But the credit for the first systematic study of the ecological implications of
the humidity response surely goes to Otto Lange and his co-workers, who,
with cultivated and wild species in the Negev Desert, examined the way in
which the compromise between the needs to take up CO2 and minimize
in Dry Air
I.R. Cowan
10.1 Introduction
The first observation that stomata tend to close in dry air seems to have
been made by one whose name is not usually found in reviews of stomatal
physiology. Joseph Banks (1805) wrote that the pores in the stems of wheat,
"which exist also on the leaves and glumes", are shut in dry weather and
open in wet. The context is an essay on the cause of mildew in wheat, in
which Banks supposes that the infection takes place through the stomata.
He presumed the pores are "a provision intended no doubt to compensate,
in some measure, the want of locomotion in vegetables" and remarked that
"A plant cannot when thirsty go to the brook and drink: but it can open
innumerable orifices for the reception of every degree of moisture, which
either falls in the shape of rain or dew, or is separated from the mass of
water always held in solution in the atmosphere .... "
Nearly a century later, Francis Darwin (1898) speculated on the role of
stomatal response to humidity in the following passage, from which the title
of this article is taken. "Some interesting questions arise as to the mode of
action of the guard cells in dry air. Can we look at guard cells as senseorgans which, when the leaf is threatened by want of water, perceive the
danger before the rest of the leaf? This idea is not wholly fanciful. Stahl has
given some reason to believe that the transpiration of the guard cells is
especially active; in dry air, or with a diminished water-supply, they should
therefore be the first to suffer." In the work to which Darwin refers, Stahl
(1894) suggested that the most important step in the "Emancipation" of
plants from high atmospheric humidity lay in the capability of stomata to
close. Ernst Stahl, who qualified as lecturer in Wiirzburg before moving to
Jena, was an accomplished ecophysiologist with an ingenuity that will be
indicated later.
The idea, then, that stomatal response to atmospheric conditions promoting transpiration is adapted to avert impending danger is quite an old one.
But the credit for the first systematic study of the ecological implications of
the humidity response surely goes to Otto Lange and his co-workers, who,
with cultivated and wild species in the Negev Desert, examined the way in
which the compromise between the needs to take up CO2 and minimize