iv.] VARIOUS THEORIES, &c. 81 



to see that a slight pressure forced it into all the 

 cavities of the alburnum exposed by the section, and 

 even through the bordered pits, to a height of ten mm. 

 In this way he blocked up the lumina of the tracheides , 

 and allowed the cacao-butter to congeal ; he then cut 

 a clean surface exposing the clean-cut walls of the 

 tracheides. A pressure of sixty an. of mercury failed 

 to force water through, whence Elfving concluded that 

 apart from any possible molecular movements of 

 water imbibed in the cell-walls) the rapid currents 

 of water in the wood take place through the cavities 

 and not in the substance of the cell-walls. 



Elfving then goes on to discuss some other 

 phenomena, showing that water is held in the vessel 

 of A ristolochia, for instance, by exactly the same force 

 as it is held in a capillary tube of like calibre, and that 

 the tracheides and bordered pits are very impervious 

 to air. 



A piece of wood 3 cm. long allowed water to 

 pass easily under a pressure of i cm. of water, 

 whereas the pressure of a column of mercury twenty 

 cm. high failed to drive air through. Now since one 

 cm. of water exerts a pressure roughly equal to y^Vir 

 atmosphere, we have to conclude that the slightest 

 rise of pressure due to the expanding of an air bubble 

 in a tracheide, will drive water through. 



G 



