350 PHYSIOLOGY 
conditions. Nor can root pressure be invoked even as an aid. Foi 
unless maximum turgor can be attained no extrusion of water from 
cortical cells is possible. 
If a boy could push a wagon while the horse walked, he would be unable to push 
as soon as the horse's speed exceeded his own. If he clung to the wagon, he would 
be merely a drag, though if he ran he would be less of a drag than if he made no 
exertion. The transpiration horse often goes too fast with the water wagon for the 
root pressure boy to push. Then his grip is broken at once and he is no drag on 
the load, for root pressure cannot even hold on like the boy and " help " by not 
being wholly a drag. 
Atmospheric pressure. Atmospheric pressure has been invoked as 
an explanation. It is found that the gases which develop in the tracheae 
are often under a pressure less than one atmosphere. Indeed they 
develop there readily because this is the case. The tracheae, it must 
be remembered, are dead cells; their lumina therefore are as free to be 
occupied by gases as are intercellular spaces. Whenever the concen- 
tration of gases dissolved in a free liquid exceeds the amount normal 
at one atmosphere pressure, the gas particles escape from solution and 
form bubbles. 
This happens when any bottle of liquid " charged " with COg is opened. The 
gas is dissolved in the liquid under a pressure greater than one atmosphere; on un- 
corking it the pressure is reduced immediately to that of the outer air, the gas flashes 
at once into bubbles, and portions of the liquid are often forced out of the bottle 
by the violence of effervescence. 
Bubbles would inevitably form in the water of the tracheae, whenever 
that water has the pressure on it reduced below one atmosphere. If this 
pressure were equal to half an atmosphere, it is argued that such tension 
could " lift " water about 5 m. So it could, if the lower end of the water 
columns were open to the pressure of the atmosphere and there were no 
resistance. If one took away half an atmosphere of pressure from the 
upper end of a water column and left a whole atmosphere of pressure to 
act on the lower end, of course the water would rise to the point of equi- 
'ibrium. But these conditions do not exist in the plant. Evaporation 
may reduce the pressure on the water in the tracheae, but the lower end 
of the water column is not open. The living cells of the root cortex are 
interposed, and water cannot be driven through them by a difference of 
half an atmosphere or even a whole atmosphere of pressure; nor has 
the pressure in the tracheae ever been found to fall to zero. If it 
were zero, and there were no resistance to the movement, water could 
be pressed up to a height of only 10 m., a small fraction of the 100 m 
