60 THE OAK 



various groups of smaller vessels the groups being 

 rounded, or in radial rows, or curved or oblique rows. 



Successive sections prove that the vessels in the 

 bundle change in number i.e. there are fewer when 

 passing from stem to leaf. A vessel may end in an 

 interpectinating, pointed, terminal cell ; or it may 

 branch, as it were, dichotomously, owing to fusions with 

 other similar elements ; or such a fusion may occur 

 lower down, the original vessel ending blindly. 



In the vicinity of the reticulated and first pitted 

 vessels, following on the spiral vessels, we find libriforrn 

 fibres, tracheids, wood parenchyma, and secondary 

 rays of parenchyma ; the tracheids are especially in 

 the neighbourhood of the vessels (see fig. 14). 



The tracheids are long cells with gradually taper 

 ends, and the walls rather thick but by no means ob- 

 scuring the lumen ; on the walls are numerous, usually 

 elongated, oblique or horizontal bordered pits. These 

 pits occur whether the next element is a tracheid, a 

 vessel, or fibres or cells of any kind (fig. 16, tr). 



The length of the tracheids varies, and the diameter 

 is also variable. 



The libriform fibres are also long cells, but often 

 more pointed at the ends, and their very thick walls 

 almost obliterate the lumen (fig. 16, /) ; their length is 

 about that of the tracheids, but slit-like, small, simple 

 pits are rare on their walls. In the wood of later years, 

 however, the lengths may be different. 



There are also elements which stand midway between 



