248 HOW CROPS GROW. 
latter, in forcing their way against considerable pressure, 
often expand around, and partially envelope, the particles 
of earth. 
Imbibition of Water by the Root.—The degree of 
force with which active roots imbibe the water of the soil 
is very great, is, in fact, sufficient to force the liquid upward 
i into the stem and to exert a con- 
tinual pressure on all parts of the 
plant. When the stem of a plant 
in vigorous growth is cut off near 
the root, and a pressure-gauge is 
attached to it as in fig. 43, we 
have the means of observing and 
measuring the force with which 
the roots absorb water. The pres- 
sure-gauge contains a quantity of 
mercury in the middle reservoir, 
6, and the tube, c. It is attached 
to the stem of the plant, p, by a 
stout india-rubber pipe, g.* For 
accurate measurements the space, 
a and 6, should be filled with wa- 
ter. Thus arranged, it is found 
that water will enter a through 
the stem, and the mercury will 
rise in the tube, e, until its pres- 
Fig. 43. sure becomes sufficient to balance 
the absorptive power of the roots. Hales, who first ex- 
perimented in this manner 140 years ago, found in one 
instance, that the pressure exerted on a gauge attached in 
spring-time to the stump of a grape vine, supported a 
column of mercury 324 inches high, which ; is equal to a 
column of water of 364 ft. Hofmeister obtained on other 
plants, rooted in pots, the following results: 
* For experimenting on small plants, a simple tube of glass may be adjusted 
to the stump vertically by help of a rubber connector. 
