96 TRANSPIRATION AND ASCENT OF SAP ch. 



as they are shown to do, an immense resistance to flow- 

 is inexplicable on any view which regards the water as 

 being forced through the stem. Viewed, however, in the 

 light of the tension hypothesis, this structure becomes a 

 most beautiful adaptation to confer stability on the 

 tensilely stressed transpiration stream, and one which 

 transforms the water, despite its mobility, into a sub- 

 stance which is stable while sustaining very great stresses, 

 just as if it were a rigid body. True, the tensile stream 

 experiences the resistance opposed by the numerous walls, 

 but the presence of the partitions, conferring, in the manner 

 just pointed out, a new property on the water, renders 

 available such an enormous source of energy at the evapo- 

 rating surfaces in the leaves for the lifting of the sap, that 

 the amount of energy which is spent in overcoming the 

 resistance opposed by the walls is relatively insignifi- 

 cant. 



The elongated form of the conducting elements secures 

 that the resistance shall be small consistently with the 

 stability of the water ; for, of course, if the tension is great, 

 a bubble in a long tube renders a larger portion of the 

 conducting tissues useless than does one confined in a 

 short vessel ; but on the other hand, when the long tube 

 is completely filled, it transmits more readily than if it 

 were subdivided into a number of tracheids. Hence we 

 may regard the tissue formed of long vessels as the path 

 of the most rapid part of the transpiration current when 

 the plant has an abundant supply of water, while the 

 tracheids transmit the slowly moving water and continue 

 in function even when supplies are very limited. It is 

 also evident that the small cross section of the tubes, 

 though introducing resistance, is most essential. In this 

 way each bubble which is formed occupies only an infini- 

 tesimal part of the cross-section of the whole water current. 



The structure of the walls themselves is also in com- 

 plete harmony with the tension hypothesis, and finds its 



