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 k’, 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 flow. 
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 he considered. Without an adequate supply of moisture 
