K— BOTANY 203 



Phloem and bark can be easily removed, these tissues coming away at the 

 plastic cambium layer without any damage to the differentiating vessels 

 within. These vessels, and the older wood within, can now be pierced 

 with a sharp knife through a drop of Indian ink placed on the surface of 

 the cambium. The newly differentiating vessels are full of liquid at 

 first and do not inject at all when so cut ; very often instead sap flows 

 freely from them, though they never give off a fine spray when punctured, 

 as the differentiating tracheae of the softwoods sometimes do. While 

 they are thus found full of liquid, the vessels of the old wood immediately 

 beneath them will be found to inject freely, which may be due to the 

 withdrawal of water from these vessels by the differentiating elements 

 forming outside them. This suggestion is supported by observations 

 on the ash, in which the vessels are extraordinarily long, so that when 

 they are cut open they cease to function throughout a great length of 

 the tree. Beneath the surface of such punctured differentiating 

 vessels the old wood has often subsequently failed to inject, whilst 

 beneath undamaged differentiating vessels on either side it injected very 

 freely. 



A little later in the year (by June i this year in Leeds), the differ- 

 entiated lignified vessels in the newly forming ring of wood inject freely, 

 and from then onwards, as von Hohnel first pointed out, this evidence of 

 a state of tension in the contents spreads gradually inwards, the vessels of 

 progressively older and inner rings of sap wood injecting as the summer 

 progresses. 



The Contents of the Wood. — Nowadays it is usually assumed that the 

 tracheal elements remain full of water although their contents are under 

 tension. In this case, each day that water loss from the tree by evapora- 

 tion exceeds water entry by absorption, the tension must mount in the 

 tracheal system and should reach extraordinarily high values. This 

 possibility must now be more closely examined. In the first place it is 

 clear that the structure of the tracheal system is such that air will not 

 readily enter to displace the water, although air at approximately 

 atmospheric pressure is present in the intercellular spaces that form 

 a continuous system along the flanks of the rays, and which are in 

 immediate contact with the tracheae and in communication, through 

 the cambium, with the intercellular system outside and with the 

 outside air. 



Air. — The only pores in the water-saturated wall of any element of the 

 wood, including the hardwood vessel, through which gas might enter 

 are the minute holes in the thin primary walls that run across the pits, 

 which were originally filled by the plasmodesma strands. In the soft- 

 wood Bailey has recently demonstrated the passage of gas and fine sus- 

 pensions through these pores, and concludes that in Larix laricina they 

 vary in diameter from 3[i to o-5(i.. The smaller pores here would need 

 pressures of 5 • 8 atmospheres on either side of the wall in order to drive 

 air through to displace water. Bailey actually drove air into the tracheids 

 of Larix by using pressures below 3 atmospheres. In the hardwood the 

 pores in the pits are certainly smaller : A. Meyer estimated their diameter 

 at o-i5(ji, whilst Renner estimates them at less than OSV-- i" actual 



