CELL-CONTENTS AND FORMS OF CELLS. 275 



The tracheids are intermediate in character between tracheae 

 and libriform, resembling the former in possessing bordered pores 

 (Fig. 145) and scalariform thickenings; and the latter in being 

 true cells, which are usually elongated and quite thick-walled, 

 the walls giving distinct reactions for lignocellulose with phloro- 

 glucin or aniline sulphate. 



One of the chief characteristics of tracheids is the BORDERED 

 PORES (Fig. 145). These differ from simple pores in that the 

 wall surrounding the pore forms a dome-shaped or blister-like 



3 



S 



FIG. 145. 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 Vog!. 



protrusion into the cell. On surface view the pores are either 

 circular or elliptical in outline, the dome being circular or, if the 

 pores are numerous and arranged close together, more or less 

 polygonal (Figs. 143, 144). 



The number and distribution of bordered pores in the Coni- 

 ferae are quite characteristic for some of the genera, and may be 

 studied in any of the pines, the pores being most numerous in 

 the radial walls (Fig. 69). 



