THE WALLFLOWER 53L 



The wood in the Wallflower consists of three kinds 

 of elements, which are best studied in longitudinal 

 section (see Fig. 22). 



The Wood or Xylcm 



I. The Vessels 



These are long, continuous tubes, which extend 

 through the plant for considerable distances without, 

 any interruption. When mature, they appear empty, 

 containing no protoplasm, but only water and air.. 

 The vessels have very peculiar markings on their 

 walls, which are due to their being unequally thick- 

 ened, some parts of the wall growing much in 

 thickness, while other parts remain quite thin. In 

 the vessels nearest the pith the thickening generally 

 takes the form of a spiral thread, which winds along 

 the inside of the wall. The parts of the wall between 

 the coils of the spiral are quite thin. The structure 

 of one of these spiral vessels is exactly like that of an 

 incliarubber hose with a stiff spiral wire coiled inside 

 it to prevent its collapsing. In the innermost vessels 

 the spiral is always loosely coiled. This is because 

 the innermost vessels are the first to be formed, and 

 their walls become thickened before the tissues have 

 stopped growing in length, so that as growth goes on 

 the spiral gets drawn out. 



Sometimes, instead of the continuous spiral thread,, 

 we find isolated transverse rings round the vessel, 

 which is then called annular. This form also admits. 



