THE WATER-RELATION 



89 



been confirmed by determinations of the osmotic pressures in the saps oi 

 various leaves. These pressures have always been found adequate to resist 

 the transpiration-tension." 



On the other hand there is also a propellent factor which may act 

 from below. If a rooted stem be cut a few inches above the soil on a 

 warm day, and the cut surface be immediately examined with a lens, 

 it will appear dry. A drop of water 

 placed upon it will at first be absorbed. 

 But in a short time it will be found that 

 water exudes from the lower cut surface, 

 coming out from the cavities of the vessels. 

 If a tube be fitted upon it with a tight joint, 

 the fluid exuded will collect and rise in it. 

 In the case of a potted Fuchsia plant 

 about three feet high it has been seen to 

 rise in a few days to a height of as much 

 as 20 feet. Or a tube may be fitted as 

 in Fig. 66, by which the pressure from 

 below may be measured in inches of a 

 column of mercury forced up by it. This 

 demonstrates Root-Pressure, a second 

 factor which may influence the movement 

 of the Transpiration-Stream. These two 

 factors can act simultaneously, though 

 both vary independently. In the daytime 

 normally the suctional influence is the 

 greater, and this explains the fact that 



When first CUt the Surface Of the Stump root-pressure. The .tube g is fitted ion 



r the cut stem s. Water is absorbed 



appears dry, and water exudes only after by the roots and forced into the tube, 



and the pressure can be measured in 



SOme time has elapsed. The COntlllUOUS terms of the height of a mercury 



, . column (Q), which it is able to raise. 



movement ot the Iranspiration-btream is s. 



chiefly due to these two factors, though it is possible that others 



participate with them in the case of tall trees. 



The water of the Transpiration-Stream is not pure water. This 

 may readily be shown by evaporating some of the water extruded 

 upon a clean glass surface, when a residuum will be left. It is com- 

 posed of certain salts, together with sugar, and other substances. The 

 most important contents as regards the general nutrition of the plant 

 are the dissolved salts, though organic substances may also be present 

 in solution. A prominent example is Maple-sugar in the sap of the 

 Maple in spring. When the Transpiration-Stream moves up the plant 



FIG. 66. 

 Arrangement for demonstrating 



