THE STEM 173 



tact with the atmosphere by means of a transverse 

 section, the equilibrium between the internal and 

 external atmosphere is suddenly established. This is why 

 it is necessary to cut through the stem under mercury. 

 However quickly we dip a cut stem into mercury, we 

 shall always remain without any result whatever. 

 But if we leave the end of a cut stem sunk in mercury 

 for some time, we shall find that mercury will begin 

 to rise in its vessels. This experiment will give us an 

 answer to the second of the questions raised above — 

 how is the origin of this rarefied atmosphere to be 

 explained ? Its explanation is as follows. Leaves 

 evaporate water, and hence more concentrated solu- 

 tions of substances contained in them are formed in 

 their cells. These solutions, as we have already seen 

 (fig. 46 on the right), draw fresh quantities of water 

 from the neighbouring cells, and thus from cell to cell 

 absorb the water stored in the vessels. But if water 

 be drawn out of the vessels, the air of the bubbles 

 alternating with it takes its place, and increases in 

 volume, i.e. becomes rarefied. As a result of this rare- 

 faction, a fresh quantity of water is absorbed by the 

 vessels from the cells of the root. The truth of this 

 inference can be proved by direct experiments. If we 

 make a section of the tip of a stem (bearing leayes) 

 transparent enough to be placed in a drop of water 

 under the microscope, we shall be able to learn the 

 following facts. If small particles of a powdered sub- 

 stance are suspended in the drop of water, we shall 

 notice them tending towards the apertures of the 

 vessels and flowing right through them. The bubbles 

 that we see in the vessels will either decrease in volume 

 with the decrease of evaporation from the leaves, or 

 else increase, i.e. the air will become rarefied, with 

 the increase of evaporation from the leaves (fig. 49, 

 II, a and b).^ 



1 In Fig. 49 II, a and b, two consecutive stages of one and the same vessel 



