72 Proceedings of the Royal Irish Academy. 



Experiment C. in Table III. is subject to a correction for the 

 elasticity of the branches' conduits. In determining the amount of 

 "water transpired, the vessel beneath was placed in a position before 

 the pressure was raised in the glass cylinder and removed for its second 

 weighing, while the pressure was still maintained. Consequently 

 some water was squeezed back from the conduits, owing to their 

 elastic yielding to the pressure, and remained in the vessel, diminishing 

 the amount of transpiration observed. In order to estimate how much 

 ought to be allowed for this, an experiment was made in which the 

 same branch was raised to a pressure of 6 atmospheres for ten minutes. 

 While the pressure was maintaiued a weighed vessel containing some 

 water was supplied to its protruding end, and then the pressure was 

 lowered to normal atmospheric pressure. After ten minutes the vessel 

 was reweighed and was found to have lost 0408 gr. due to the elastic 

 recovery of the conduits. When this allowance is made in experi- 

 ment C, Table III., the amount transpired becomes 0-219 gr., instead 

 of 0-111 gr.i 



In order to determine whether this elastic contraction of the con- 

 duits occurred chiefly in the conduits of the stem or leaf, experiments 

 were made in which a branch was first exposed to a pressure of 

 6 atmospheres for ten minutes, and while the pressure was still 

 maintained, a weighed quantity of water was supplied to its lower 

 end which protruded from the high-pressure apparatus. The pres- 

 sure was then immediately lowered, and the branch was left to draw 

 up water from below for ten minutes by means of its elasticity, and 

 the amount which is drawn up is measured by a second weighing. 

 When this amount is compared with the amount drawn up in a similar 

 experiment with the same branch when all the blades of the leaves 

 are removed, it is found that the former is very much greater than the 

 latter quantity. Thus with a branch of Tilia americana bearing 11 



1 Tlie fact that the presence of CO2 in contact with the leaves modifies so pro- 

 foundly their power of drawing up water against pressure, appears as an additional 

 argument for believing that the osmotic properties of the mesophyll-cells is a more 

 important factor in transpiration than the imbibition or capillary phenomena of the 

 ceU-wall. For we can hardly believe that the solution of this gas in the water 

 could possibly reduce the surface-tension sufficiently to account for the difference 

 observed ; whereas it is readily comprehensible that the presence of CO3 would 

 greatly reduce the osmotic pressure of the cells by introducing changes in the pri- 

 mordial utricle (possibly owing to the exclusion of oxygen and consequent intra- 

 molecular respiration), or even by forming insoluble substances with the solutions 

 in the vacuoles. 



