iv.] VARIOUS THEORIES, &C. 67 



they transpire in one season quantities amounting to 

 many times the volume of the whole plant. 



Taking the wood of Conifers as being the simplest 

 and most thoroughly studied, and choosing that of 

 the yew because it is devoid of resin-canals, he 

 points out that the tracheides are closed cavities 

 the membranes of the bordered pits forming a com- 

 plete septum and hence a piece of yew wood may 

 be employed to filter off fine particles. There being 

 no capillary tubes here, we cannot entertain the idea 

 of a capillary ascent : moreover, even in Dicotyledons, 

 the sectional areas of the lumina of the vessels are too 

 large to admit of an ascent by capillarity beyond a 

 few yards at most. 



But now comes a weighty argument. At the time 

 when transpiration is most active in the summer, and 

 therefore when most water is passing through the 

 wood, the cavities of the tracheides and vessels are 

 not full of water, but contain very little, and the 

 vessels may even (so Sachs asserts) be empty : hence 

 the wood floats on water, which it would not do 

 unless considerable air-cavities existed. The presence 

 of the air can also be proved by warming the wood, 

 and the phenomenon of " water-logging " depends on 

 the gradual filling of the cavities by water, which 

 soaks in and displaces the air. 



F 2 



