THE STEUCTUEE OF THE EOOT 



II 



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

 long, narrow, and very much 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. This addition 

 of substance to the 

 original cell - wall 

 (primary mem- 

 hrane), 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, 

 Fig. 2 g). In 

 some cases the 

 thickening is net- 

 like (reticulate), 

 ladderlike (scalari- 

 form), or presents 

 the appearance of 

 mary wall (Kg. 2 



^Tl> 



nflrl 



|>=^V^ I 



a spiral band on the inside of the pri-. 

 ^). According^ to the manner in which/ 



