270 HOW CROPS GROW. 



The pressure-gauge contains a quantity of mercury in 

 the middle reservoir, J, and the tube, c. It is attached 

 to the stem of the plant, p, by a stout india-rubber 

 pipe, <?.* For accurate measurements, the space a and 

 b should be filled with water. Thus arranged, it is found 

 that water will enter a through the stem, and the mer- 

 cury will rise in the tube, e, until its pressure becomes 

 sufficient to balance the absorptive power of the roots. 

 Stephen Hales, who first experimented in this manner 

 (1721) 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^ feet. 

 Hofmeister obtained on other plants, rooted in pots, the 

 following results : 



Bean (Phaseolus multiflorus) 6 inches of mercury. 



Nettle 14 " " 



Vine 29 " " 



The seat of absorption Dutrochet demonstrated to be 

 the surface of the young and active roots. At least, he 

 found that absorption was exerted with as much force 

 when the gauge was applied to near the lower extremity 

 of a root as when attached in the vicinity of the stem. 

 In fact, when other conditions are alike, the column of 

 liquid sustained by the roots of a plant is greater the less 

 the length of stem that remains attached to them. The 

 stem thus resists the rise of liquid in the plant. 



While the seat of absorptive power in the root lies 

 near the extremities, it appears from the experiments of 

 Ohlerts that the extremities themselves are incapable of 

 imbibing water. In trials with young pea, flax, lupine 

 and horseradish plants with unbranched roots, he found 

 that they withered speedily when the tips of the roots 

 were immersed for about one-fourth of an inch in water, 



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

 adjusted to the stump vertically by help of a rubber connector. 



