Transpiration and the Ascent of Sap. 



43 



S c h w e n d e n e r 's figure, like islands in a river. Hence it is evident 

 that it would be impossible to sever the continuity of the water 

 in the conducting tracts, 



i. e. to prevent evapora- 

 tion above from trans- 

 mitting a pull to the 

 water in the roots, unless 



tracheae containing 

 bubbles were to form in 

 some place an unbroken 

 diaphragm across the 

 conducting tissues of the 

 stem. 



From this examina- 

 tion it appears that, un- 

 . less an exceedingly large 

 number of the conducting 

 tubes contain air and 

 are arranged in a spe- 

 cial manner, there is no 

 likelihood of the tensile 

 column being broken. On 



the other hand the 

 amount of water trans- 

 mitted in the stream will 

 be affected by the number 

 of tracheae which con- 

 tain bubbles and which 

 are consequently put out 

 of action in the trans- 

 mission of water drawn 

 upwards under tension. 

 Hence it is of interest 

 to enquire into our state 

 of knowledge as to the 

 air-contents of the con- 

 ducting tracts. 



Results like H a r - 



tig's ^), where the amount 

 of air present is estimated 

 as a percentage of the 

 volume, cannot be utilized 



Fig. 4. Diagrammatic representation of a tangential 

 section of the wood of a conifer during transpiration. 

 Tlie shaded lamina are supposed to be filled with 

 water; those which are left unshaded are occupied 

 by bubbles. About 50 "^/o contain water in the case 

 figured. 



^) E. Hart ig, Ueber die Vertheihmg der organischen Substanz, des Wassers 



