BORDERED PITS 37 



to one side or the other, the torus closes up the aperture of the 

 dome, and thereby any passage of water or of air from one 

 vessel to the other is prevented. Such displacement will occur 

 when the pressure in one vessel differs from that in the adjacent 

 one, so that the bordered pits operate as safety-valves whereby 

 differences of pressure in the wood become localised. 



The conduction of water in the higher plants is mainly effected 

 by means of vessels, but another kind of conducting element, 



O i 



FIG. 1 8. Tracheids. A, Spiral type, from the leaf of Dog's Mercury 

 Mercurialis). B, Bordered-pitted, from the stem of the Scotch Fir 

 (Pinus). b, border of bordered pit ; wj., middle lamella ; p, aperture 

 of bordered pit ; tr., tracheids. 



the tracheid, which is likewise dead, is also found in the wood 

 of thicker branches and, more commonly, in leaves. Each 

 tracheid is formed from a single cell which is more or less 

 elongated in form (i.e. prosenchymatous), and usually bears 

 bordered pits (Fig. 18, B) or spiral thickening (Fig. 18, A). The 

 wood of the Fir and other Conifers is entirely made up of long 

 tracheids, similar in appearance to fibres, but bearing pronounced 

 bordered pits (Fig. 18, B). Very narrow bordered pits appearing 

 as oblique slits are not uncommon in fibres proper. 



