328 PHYSIOLOGY 
between them permits free diffusion of the water vapor into the outer 
air. Conversely, the guard cells become flaccid with scant water, 
straighten elastically, and practically close the slit. This sort of adjust- 
ment is held to " regulate " the transpiration, permitting it when water 
is abundant, reducing it when the supply is inadequate. Yet if the 
assumption of the existence of special osmotically active substances in 
the guard cells were correct, they should be the last to feel the slackening 
of the water supply; and so one must assume further that they are ad- 
justed to water much as are the other green cells of the leaf an assump- 
tion which is hardly justifiable in view of their position and connections. 
Of course the immediate effect of the reduction in area of the stomatal 
slits is to reduce the amount of vapor diffusing through them. But 
this in turn would increase the relative humidity of the internal atmos- 
phere (i.e. that of the intercellular spaces), would cause the accumula- 
tion of a " head " of pressure, so to speak, that would accelerate the dif- 
fusion through the narrower slit, and the system would tend to reach an 
equilibrium again. Thus the closure of the stomata is rendered par- 
tially or wholly ineffective. Were the internal atmosphere saturated 
with moisture when stomata are open (as has been assumed), the closure 
could not have this effect. But this assumption has not proved correct. 
Other changes in the external world (i.e. stimuli) affect the guard cells. 
Of these light is the most notable. In general the guard cells curve in 
light and straighten in darkness; the tendency, then, is for the stomata 
to open at a time when the evaporation is greatest and to close when it 
is least. 
It is difficult to reconcile the facts with the commonly accepted view 
that the stomata are " delicately balanced valves " which adjust trans- 
piration to the " needs " of the plant. 
If the logic on which that idea rests were valid, it would prove rather 
that the stomata regulate the admission of gases, since any diminution 
of the size of the slit must diminish the amount of CO 2 admitted to the 
air passages, no " backing up " and accumulation of a " head " of pres- 
sure being possible in this case, whereas it does occur with water vapor 
diffusing from the plant. In the absence of any apparent advantage 
in regulating the movement of gases, and the " need " of some control 
over evaporation, it has been assumed that the stomata are able to 
adjust matters so that enough water will flow through the plant, carry- 
ing with it needed salts, while at any time these governors can check 
loss when danger threatens. Many cases, however, have been reported 
