THE ANGIOSPERMAE : STEMS 



899 



is no invariable distinction of size, though normally the expanded vessels are 

 larger than the tracheids of the same wood. While vessels are normally the 

 channels of long distance transport, tracheids serve chiefly for short distances 

 and for water storage. Offshoots of the main bundles, including the leaf 

 traces and the leaf veins, consist almost entirely of tracheids. This distinction 

 of function implies an advanced state of specialization and is characteristic 

 of the Dicotvledons. 



Fig. 882. — Robhiia psetidacacia. Transverse section of a portion of 

 secondary wood showing large vessels filled with thin-walled tyloses. 



Fibres play a large part in the make up of the secondary wood (Fig. 883). 

 Firstly there are the fibre-tracheids, thicker walled than the true tracheids 

 but with the same bordered pits, and in some cases not sharply distinguish- 

 able. These grade through intermediate elements, increasingly narrow, 

 thick-walled and elongated, to true libriform fibres. 



Libriform fibres in the wood are often as much as seven times as long as 

 the cambial initials. Measurements show variations between 0-3 and i -3 mm. 

 Nevertheless this is small compared with the bast-fibres, which are seldom 

 less than i-o mm. long and may reach 77 mm. in Urtica (Nettle), or even 

 40 cm., i.e., 16 in., in Boehmeria nivea (Ramie). In both cases, however, the 

 elongation during differentiation is very considerable. As it takes place in 

 regions where general growth in length has ceased, it has been held that the 

 elongation of the individual cells must imply slipping or sliding of the cells 

 on each other, and this has been called sliding growth. The idea is open to 

 some anatomical objections, but it cannot be said that the alternatives suggested 

 are very convincing. One counter-argument, usually regarded as weighty, 

 is that the pits in the walls of adjacent fibres show a normal degree of corre- 

 spondence, which is considered to be impossible if the walls have slipped 

 during growth. It is not impossible, however, that this correspondence may 

 be secondarily established, since it is also found in the pits between adjacent 



