H4 TIMBER AND SOME OF ITS DISEASES. [CHAF. 



exist in the cavities of the tracheides, and that the 

 system which has to be examined, consists of fine 

 capillary tubes plunged below into cells which absorb 

 water (root-parenchyma, &c.) and above into cells 

 which give off water by evaporation (mesophyll of 

 the leaf), and accompanied on their course by cells 

 of wood-parenchyma and medullary-rays, the latter 

 supplying the cortex with water. The whole con- 

 ducting mechanism moreover is surrounded by the 

 cambium. The up-taking and giving off of water is 

 accomplished through the bordered pits, and the 

 capillary system is, as we have seen, impermeable to 

 air. It should also be noted that by far the majority 

 of so-called vessels are really tracheides. 



So long as the closing membranes of the bordered 

 pits are not stretched, the tracheide is a closed system : 

 pressure on the membrane results in the passage of 

 water in the direction of the pressure, the membrane 

 returning to its original position elastically and pre- 

 venting the back-flow. Hence the water which 

 traversed the membrane is held in the capillary space 

 above. 



There is, said Scheit, no question of the sum of the 

 pressures of the columns in superposed tracheides, 

 because each column is on the one hand held up by 

 capillarity, and on the other, could only exert pressure 



