130 TIMBER AND SOME OF ITS DISEASES, [chap. 



tracheide, and vice versd, through the large permeable 

 closing membrane. 



At a given time, any of these tracheides contains 

 air and much water : the cell contains protoplasm and 

 osmotic substances. 



Now suppose the medullary ray-cell to absorb water 

 osmotically from the tracheides^ along its two sides, 

 becoming more and more turgid, and the contents 

 pressing the thin membranes outwards into the 

 cavities of the tracheides : this does not involve any 

 change of pressure of the air-bubbles in the tracheides, 

 for the membrane is pushed in towards the cavity as 

 water is withdrawn. 



The turgesence of the medullary ray-cell at length 

 attains a maximum. Then follow the changes which 

 lower the osmotic power of the cell contents, and 

 some of the water is driven out under pressure — 

 simply because the cell contents can no longer 

 retain it. This forcible exfiltration of water will 

 increase the pressure on the air-bubbles in the 

 tracheides whence the water came, and so if in any 

 neighbouring tracheide there is less pressure, part 

 of the water absorbed from, say eight tracheides, will 

 escape into that one. Moreover, since the pits are 



^ There may be eight tracheides or more flanking any one medullary 

 ray-cell. 



