2i6 The Living Plant 



tions in this, that it involves the activity of cells which are alive; 

 and herein also it meets its greatest difficulty, because, accord- 

 ing to some experimenters, when the living cells are killed by 

 suitable methods, the water continues to ascend, at least for 

 some time. Therefore, they say, this cannot be the explanation. 

 But others are not convinced that the cells are really all killed in 

 these experiments, and hold that this explanation is substantially 

 correct. 



6. Traction. — This, the most recent explanation, has been 

 worked out by a botanist, Dixon, and a physicist, Joly, working 

 in collaboration, and is often known by their name. It maintains, 

 in brief, that water in very thin threads holds together, by the 

 force of its own internal cohesion, with a tenacity sufficient to make 

 it as strong as a solid fiber or wire; wherefore the thin threads 

 of water in the ducts can actually sustain their own weight for 

 a length as great as the height of the tallest trees. These threads 

 being practically continuous from the tips of the roots to the 

 cells of the leaves, hang, as it were, from the leaf-cells, into which 

 they can be lifted by any power that can remove the water from 

 those cells. This power is supplied by the energy of evaporation 

 in transpiration, which latter process, therefore, lifts or drags 

 the water threads up the ducts much as a man on a roof would 

 pull up a rope from the ground. On this view the energy which 

 raises the water in the tree is the same which lifts it to the clouds. 

 This theory finds its chief difficulty in the lack of complete demon- 

 stration that the water can thus cling together in threads of such 

 great length, and it has not been universally accepted. 



It sometimes appears as if the extent of our knowledge of any 

 subject were inversely proportional to its importance. At all 

 events we found this to be true of the structure of protoplasm, 

 and it also seems true of this subject of sap ascent. And at 

 present there is a pause in the advance of our knowledge thereof. 

 With this subject, as with others, we find out everything that 

 existent methods of investigation can yield, then turn for a time 



