THE STRUCTURE OF THE ROOT 



narrow or wider tubes (vessels), while others take the form of 

 long, narrow, and very mucli thickened fibres (wood cells). 



The vessels are 

 formed of super- 

 posed cells, which 

 have become joined 

 by the loss of their 

 transverse walls, 

 and thus form con- 

 tinuous canals. The 

 lateral walls be- 

 come thickened by 

 the addition of 

 woody substance 

 on their inner sur- 

 face. Tins addition 

 of substance to the 

 original cell - wall 

 (primary mem- 

 brane), which only 

 becomes visible as 

 the vessels become 

 old, constitutes the 

 secondary mem- 

 brane, and appears 

 sometimes as a 

 fairly continuous 

 thickening, leaving 

 certain thinner 

 places, the pores or 

 pits (pitted vessel, 

 rig. 2 g). In 

 some cases the 

 thickening is net- 

 like (reticidate), 

 ladderlike (scalari- 

 form), or presents 

 the appearance of a spiral band on the inside of the pri- 

 mary wall (Fig. 2 g'). According to the manner in which 



