CH. IV, 6] TRANSFER THROUGH PLANTS 



149 



most recently of all, a new and striking explanation has been 

 offered, with much experimental support, to the effect that 

 the water rises by traction, i.e. is drawn up in long threads, 

 as if solid, by forces acting in the leaves. This matter needs 

 somewhat fuller explanation. 



In the next chapter it will be shown that the forces of 

 ^osmotic pressure, operating in the roots, draw water from 

 the soil and give it a start up the 

 stem ; also the same forces in the 

 leaves draw water in the same 

 way from the ducts into the leaf 

 cells. Now it is found that the 

 forces thus exerted by the leaf 

 cells are amply powerful to lift 

 the water to the tops of the tall- 

 est trees if only the water in the 

 ducts would hold together in 

 threads. The new theory main- 

 tains that the water does thus 

 hold together, as if in solid 

 threads, by virtue of its own 

 internal cohesion, a property 

 which is manifest in part in the 

 surface-tension familiar to all 

 students of physics. Everybody knows that a large water- 

 drop hanging free from the under side of a glass plate can 

 be lifted with the plate, and it seems clear that the water 

 could be lifted in much larger masses if lengthened out to 

 very thin threads, as it is, of course, in the ducts. The 

 water thus pulled into the leaves by the osmotic power of 

 the leaf cells is removed from those cells by the still greater 

 power of evaporation (transpiration), the energy for which 

 is supplied by the heat of the surroundings. Thus, on this 

 theory, it is really the energy of evaporation which raises the 

 water in tall trees. But while evaporation is the principal, 

 it is not the only source of energy available, for obviously 



FIG. 104. Tangential sec- 

 tion (i.e. at right angles to a 

 medullary ray) of the Pine of 

 Figs. 102-3, but more highly 

 magnified. 



The tracheids, with ; their 

 bordered pits, are plain, as are 

 the cut ends of the medullary 

 rays, of which one contains a 

 resin canal. (From Cavers.) 



