90 VEGETABLE PHYSIOLOGY 



the two limbs to be at unequal levels. By the displacement 

 of the mercury the force of the root-pressure can be 

 estimated. A variation of the apparatus can be used, in 

 which the manometer is replaced by a glass tube bent at 

 right angles. The water will be forced through this, and 

 can be collected in a suitable receiver, and its amount 

 ascertained. 



In performing the experiment it is best to allow the 

 apparatus to stand for some time before closing the tube 

 at 7c', as, if the plant is taken while transpiration is proceed- 

 ing, the vessels of the stem will contain air at a certain 

 negative pressure, and a certain amount of water will be 

 sucked back until the vessels are full. As soon as this 

 condition is reached, the pumping action of the roots will 

 become evident, and the root-pressure will make itself 

 obvious. 



The root-pressure of various plants has been measured 

 by different observers ; an idea of its amount may be 

 gathered from the fact that a medium-sized Fuchsia in a 

 pot has been found able to send a column of water up a 

 tube of the same diameter as the stem to a height of twenty- 

 five feet. 



The activity of the roots will depend upon various 

 conditions, of which temperature, both of the air and of 

 the soil, is one of the most important. The exudation of 

 water has been observed at temperatures as low as freezing 

 point, but most plants will not show it below about 5 C., 

 and as the air becomes warmer the quantity of water given 

 off increases. Warming the soil of the pot in which is the 

 plant under observation also increases the now. 



Of other influences which exert an effect upon the 

 activity of the roots may be mentioned oxygen. Like all 

 other vital actions, the absorptive power of the root-hairs 

 depends upon their being in a healthy condition, and this 

 cannot be maintained in any protoplast without the due 

 performance of respiration. The character of the soil must 

 also be considered. Without an adequate supply of moisture 



