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siderable height, which is equivalent to the pressure sufficient 

 to support the weight of at least an atmosphere if not more, 

 so that it is no hypothetical force we have to deal with. The 

 limit of distance to which the mercury column is raised is, 

 however, soon arrived at, and a condition of equilibrium be- 

 tween the downward pressure of the mercury and the upward 

 pressure of the force is attained. No other force is present to 

 occasion this root pressure than those of absorption and filtra- 

 tion under pressure, and hence this rise of liquid to a very 

 considerable height is the expression of the absorbent activity of 

 the parenchyma cells of the roots measured in millimetres 

 of mercury; for the greater the facility with which they absorb 

 moisture in the form of water, the greater is the filtration into 

 the vessels, and consequently the greater is the flow of fluid in 

 the case of cutting off a stem near the ground. When we put 

 on the mercury manometer we oppose a substance — viz., 

 mercury, to the nitration of the water into the vessels, yet, 

 nevertheless, the active power is so great as to overcome the 

 ordinary resistance, and also to lift the mercury column as 

 well. In proportion, then, as the osmotic power of the par- 

 enchyma cells is greater, so will the root pressure be greater. 

 This movement of water effected by the root pressure is par- 

 ticularly conspicuous in the early spring, when the leaves are 

 just opening, and generally at the period of most vigorous 

 growth. When the leaves are once developed and expanded, 

 and transpiration has begun, the root pressure gradually dim- 

 inishes, the accumulated water being evaporated until, when 

 the plant transpires most, this root pressure is entirely absent; 

 for, if we make a section in summer, no escape or flow of liquid 

 can be obtained from the cut surface of the stem, though the 

 absorptive activity of the roots is still going on, and the amount 

 of fluid passing is very much greater than it was in the spring- 

 time, because the plant is continually losing water at its leaves, 

 which are transpiring. As fast, however, as the liquid is ab- 

 sorbed by the root-hairs and passes thence by osmosis and 

 filtration into the vessel-walls of the wood, it is no longer 

 forced into the cavities of the vessels to collect there and be 



