1902] RISE OF THE TRANSPIRATION STREAM 1 91 



manifestation in the single fixed set of conditions we have in the 



tracheae, they are not distinguishable. Ten years ago Bohm 



alone imagined (publicly) that capillarity could play the leading 



role in the ascent of sap. It had been tried and found wanting. 



Then it w^as named cohesion and sprang at once into popular 

 favor. 



I have been assuming that the transpiration stream dqps not 

 rise in tracheae entirely free of bubbles. The evidence for 

 this is unlimited. It was formerly taken as an established fact, 

 and the imbibition theory rested very largely upon it. Since 

 this theory collapsed, Schwendener has shown again by most 

 careful work that when transpiration is active continuous water 

 ** threads" are wanting. The experiments of Strasburgcr 

 (189 1 : 681-6), on which Dixon and Joly rest their case, were 

 obtained in this way: branches were cut, under water if pos- 

 sible, and left standing in shade or darkness for some hours, 

 until they had all the water they would hold ; then they were 

 examined and the youngest wood, which is most active in con- 



ducting water, was found to ** contain very little air," or **the 

 air could be detected with difficulty." It is easy to substantiate 

 this observation. The subject of experiment has absorbed all 

 the water it can, with the help of the whole pressure of the 

 atmosphere, so there is no opportunity for the demonstration of 

 cohesion, however abundant and continuous the water. The 

 experiment gives no clue to the water content and its distribu- 

 tion when transpiration is active and w^ater less accessible, and 

 cohesion might be of service. Hartig(i88S) and Wieler (1888), 

 among others, have shown conclusively that it is just in this 

 youngest wood that the water content is exceedingly variable, 

 dependent on the supply and demand. Von Hohnel (1879 : 126) 

 found the lowest tension in the youngest ring. And an experi- 

 ment of Bohm's, already cited, shows that in the case of Robinia, 

 one of Strasburger's few dicotyledonous subjects, active transpi- 

 ration leaves the vessel far from full of water. The following 

 experiment hits the question squarely; it also covers the other 

 question of movement past bubbles, on which a superfluous 

 amount of evidence is not yet in print. 



