TRANSPIRATION AND ASCENT OF SAP DiXON, 



421 



ties of the walls of contigiious water-filled tracheids render the water 

 throughout the stem continuous. Consequently the stress developed 

 above is transmitted around the air bubbles and draws the stream 

 past them, to use 

 ScliAvendener's fig- 

 ure, like islands in a 

 river. Hence it is 

 evident that it Avould 

 be impossible to 

 sever the continuity 

 of the water in the 

 conducting tracts, 

 i. e., to i^revent 

 evaporation above 

 from transmitting a 

 pull to the water in 

 the roots unless 

 trachea? containing 

 bubbles were to form 

 in some place an un- 

 broken diaphragm 

 across the conduct- 

 ing tissues of the 

 stem. 



From this exam- 

 ination it appears 

 that unless an ex- 

 ceedingly large 

 number of the con- 

 ducting tubes con- 

 tain air and are ar- 

 ranged in a special 

 manner there is no 

 likelihood of the 

 tensile column being 

 broken. On the 

 other hand, the 

 amount of water 

 transmitted in the 

 stream will be af- 

 fected by the num- 

 ber of trachea^ which contain bubbles and which are consequently put 

 out of action in the transmission of water drawn upwards under ten- 

 sion. * * * Although our knowledge as to the actual proportion 

 of tracheae containing bubbles during transpiration is very unsatis- 



FlG. 



