igo2] RISE OF THE TRANSPIRAl ION STREAM 273 



the films of water around the bubbles is the resistance to be 

 overcome by the movement of the transpiration stream. These 

 two problems have much in common, but this one admits of a 

 more conclusive experimental answer. According to most of the 

 evidence we have, more energy is required to overcome this 

 friction than in the actual lifting of the water. But this evidence 

 is most noticeable for the discrepancies in it — cf., e, g. Janse with 

 Strasburger or Sachs. In the most recent work on this subject, 

 Curtiss (1901) concludes that the resistance offered by a stem to 

 the flow of water at a fixed rate is not proportional to the length of 

 stem. He also finds that the same force is required to overcome 

 this resistance, whether it is applied as a pull or a push ; which 

 at most can be the case only within limits, because of the influ- 

 ence of the tension on the size of the bubbles. The rapidity of 

 the movement of water can involve no inconvenient measure of 

 friction, or the plant would provide for a slower current by allow- 

 mg the conducting tissue a greater cross section. Instead of 

 doing this the tree restricts its conducting area with tyloses, 

 '*gum/' and changes in the wood itself, whose only known effect 

 where they occur is to hinder the movement of water. In many 

 trees, as in Robinia, the specialization is most decidedly toward 

 the restriction of the transpiration stream to the youngest wood. 

 If it should be proven on the physical side that the water 

 surface in contact w^ith the bubble, in breaking half of the bonds 

 that held it in the liquid state, had acquired some of the motility 

 of a gas, the clearing up of the physics of the water surface 

 would altogether alter our attitude in theory toward the friction 

 problem. The idea that the surface of a liquid is more rigid than 

 the interior, certainly arose from faulty experiments, but the 

 truth of the matter is yet to be shown. If it should be found 

 that the surface shared the properties of gases again in their 

 relative indifferences to altitude, as Sachs suggests for the 

 water of imbibition — and as is not at all impossible with a half- 

 way vaporized surface — the calculation of the 10"' of water as 

 the sum of the liquid indices alone will be justified at once. It 

 is conceivable even that the elevation of water by surface tension 



