350 PHYSIOLOGY 



conditions. Nor can root pressure be invoked even as an aid. For 

 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 CO2 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- 

 librium. 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. 



