132 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



lifts may amount to a good deal, and account for the 

 ascent of the water in the tallest trees. 



Above all, the hypothesis explains in an intelligible 

 manner so many hitherto puzzling facts. Thus it 

 explains why, on cutting through the alburnum of a 

 Conifer, the young shoots drooped although 6o/ o of 

 the cubic contents of the tracheides consisted of water. 

 The air-pressure in the parts above the cut becomes 

 equalised, and hence there is no reason for the ascent 

 of the water, but on the contrary every reason for 

 its descent, for the suction will act downwards from 

 tracheide to tracheide, much as in Th. Hartig's 

 experiment. 



Again, the hypothesis affords satisfactory explana- 

 tions of the details of histological structure e.g., the 

 typical bordered pits are so many funnels and filters : 

 the border is the funnel, the membrane the filter, and 

 the torus acts the parts of the platinum cone used 

 to prevent rupture of the filter, the torus fitting tight 

 into the small hole when the pressure becomes too 

 great. The position of the pits, again, is explained : 

 those tracheides which stand on any one radial row 

 are practically at the same level, whereas those on the 

 next radial row will be a little higher up or lower down 

 hence water is pressed up step by step and spirally 

 round the stem. 



