MORPHOLOGY OF HIGHER PLANTS. 



187 



in the Umbelliferce. They are also found in leaves (Figs. 141, 

 142, 266, 271) and in fruits, as in the Unibelliferae (Fig. 246). 



The Stereome cell is very long, spindle-shaped, with more or 

 less thick walls provided with narrow oblique pores. ' The walls 

 consist of cellulose but may also become lignified ; the lumen is 

 narrow and mainly contains air. The stereome represents the 

 skeleton of plants and is the most important mechanical tissue, 

 being much firmer than the collenchyma. The stereome or 



: V ni y . 



Fig. 103. Bordered pores of the tracheids of the wood of Abies alba as viewed in 

 longitudinal section: m, middle lamella; v, i, middle and inner layers of walls of contigu- 

 ous cells ; C, pore-canal through which sap passes from one cell to another ; L, dome- 

 shaped cavity of pore; S, separating wall or closing membrane which is usually thickened 

 in the middle as shown at t. In older cells the separating membrane is broken as shown 

 in the lower pore in figure 2. At the right in figure 4 is shown a surf ace view of a bordered 

 pore, the dotted lines indicating the relation of the circles to. the structure of the pore. 

 After Yogi. 



strengthening cells of the cortex are commonly spoken of as " bast 

 fibers" (Figs. 104, WF, 299, 300). 



The Libriform cell is the strengthening cell of the xylem and 

 as has already been stated accompanies the tracheae. Libriform 

 cells are also spoken of as "wood fibers" (Figs. 104. BF, 299, 

 300). While the stereome cell is frequently not lignified. the 

 libriform cell is usually more or less lignified. giving strong re- 

 actions for ligJiin with anilin sulphate or phloroglucin solutions. 



