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 

 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 oif 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, 

 b y and the tube, c. It is attached 

 to the stem of the plant, jt>, by a 

 stout india-rubber pipe, q* For 

 accurate measurements the space, 

 a and #, 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- 

 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 32^ inches high, which is equal to a 

 column of water of 36 ft. Hofmeister obtained on other 

 plants, rooted in pots, the following results : 



Fig. 43. 



* For experimenting on small plants, a simple tube of glass may be adjusted 

 to the stump vertically by help of a rubber connector. 



