152 PLANT ANATOMY. 
arranged in a spiral inclining to the left (compare the subsequent 
references under Mechanical System of Tissue). A special 
Fie. 68.—Longitudinal section bhrougt. vessels with scalariform thickenings (/v).— 
Rhizoma Filicis (Berg.) ig 
form of thickening is represented by the bordered pits or 
areolated dots (Fig. 72). When the cell-wall becomes thickened 
Fic. 69.—Porous cells. 
toward the interior around a place which remains thin, a canal 
will remain open, which in form must approach that of a very 
