166 THE LIFE OF THE PLANT 



bent glass tube, if we only want to gather and measure 

 the quantity of liquid exuded; whereas, if we want to 

 find out the pressure under which the sap is driven out 

 through the section we use another tube of the form 

 shown in fig. 46 on the left. This doubly bent tube, 

 filled partly with water and partly with mercury, is 

 simply a manometer, serving to measure the pressure 

 under which the sap of the plant is exuded. The sap 

 drives the mercury before it as it flows into the tube ; 

 it is by the rise of the column of mercury in the open 

 bent part of the tube that we estimate the pressure. 

 Experiment has shown that this pressure can be equal 

 to that of a column of water thirty-six feet high, i.e. 

 water is driven out of the section so vigorously that it 

 might still come out even if a column of water thirty-six 

 feet high had been made to press upon the cut surface. 

 How can we explain this property of the root to raise 

 water to such a height ? The following experiment will 

 give us the answer. Let us take a small glass bell 

 (fig. 46 on the right, and b), close its lower opening with 

 a bladder and introduce a cork with a glass tube into 

 its neck, and sink the whole as shown in the figure into 

 a basin of water. If the glass bell also contained water, 

 there would be no interchange between the water of the 

 outer and inner vessels, so long as the level is the same 

 in both vessels, at n, because otherwise water would 

 soak through the bladder under its own pressure from 

 the vessel where its level is higher into the one where 

 it is lower. But let us suppose that a solution is 

 introduced into the inner vessel instead of water, a 

 solution of some substance found in the cells of plants, 

 say sugar, which, as we know, is found in abund- 

 ance in the roots of the beetroot, for instance. Then 

 a phenomenon rather perplexing at first sight will be 

 observed, even rather contradictory to what has just 

 been said about the tendency of water to reach a common 

 level in two vessels communicating with one another 



