88 JOURNAL OF FORESTRY 



Stone's theory assumes the passage of water through the cell walls 

 where it has been condensed from vapor. Nearly 40 years ago Elfving 

 demonstrated that quantities of water sufficient to meet the require- 

 ments of transpiration could not pass through the cell walls. In more 

 recent years the theory has been re-examined and subjected to experi- 

 mental tests by numerous investigators, especially by Dixon and Joly 

 (Annals of Botany, 1895, ix, p. 403. and Dixon on Transpiration and 

 the Ascent of Sap in Plants, 1914, MacMillan Company), and so far as 

 I know they practically all conclude that while some w^ater does pass 

 upward through the walls, the amount is not sufficient to prevent the 

 wilting of leaves when the plants are subjected to the transpiration 

 conditions of an ordinary laboratory. 



The statement of Stone that "the flow of sap from the sugar maple 

 when tapped is greater during the night," does not correspond with 

 the experimental evidence on the subject. (See Bulletin 103, Vermont 

 Agricultural Experiment Station, p. 137.) 



In regard to the reviewers' statement that the presence of air in 

 the trache?e would greatly retard the passage of water by successive 

 evaporation and condensation, I may say that the experimental evi- 

 dence of its absence or presence is unsatisfactory. I think the general 

 opinion, however, is that air (or gas) is present in variable quantities 

 dependent upon the physiological condition of the tree in all tracheae 

 more than one year old and from the fact that nature connects up 

 the newly made water channels of the year with those of the previous 

 years, it is thought that the former do not in themselves transfer 

 enough w^ater to meet the requirements. 



The membrane of the bordered pit has bothered many plant physi- 

 ologists. It probably is not as accommodating as many would like to 

 have it. Since it is not heavily lignified, water passes through it with 

 much greater ease than through a wall. A moderate flow of water 

 does not deflect it from its median position. In fact, some investi- 

 gators, like Pappehheim, for example, say that the normal transpiration 

 current does not possess sufficient velocity to force it over to a lateral 

 position. 



Finally, Dixon in liis book on Transpiration and the Ascent of Sap 

 in Plants, p. 43, states the vapor pressure theory very clearly. He sub- 

 jected it to experimental conditions and then stated his conclusion as 

 follows : "It appears that the movement upwards of water in the form 

 of vapor through the lumina is insignificant." 



I hope the above will not discourage Dr. Tiemann from investigating 



