THICKENINGS OF THE WALL. 27 



away as in the previous case, and so free communication 

 between adjacent cells or vessels is established. 



j 



FIG. 18. 



Fig. 17. Bordered pits of the thick root of Dahlia variabilis. A, front view of a 

 piece of the wall of a vessel, seen from without ; B, transverse section of the Fame 

 (horizontal, and at right angles to the paper) ; C, longitudinal section of A (vertical, 

 and at right angles to the paper) ; q, septum ; a, the original thin thickening-ridge ; 

 b, the expanded part of the thickening masses, formed later and overarching the pit ; 

 t, the fissure through which the cavity of the pit communicates with the cell cavity ; 

 at cc and >3 the corresponding front view is appended, in order to make the trans- 

 verse and longitudinal sections more clear, x 800. After Sachs. 



Fig. 18. Scalariform thickening of the walls of a vessel from the underground 

 Btem of Pteris aquilina. A, half- vessel, isolated by Schulze's maceration ; B to Z>, 

 pieces obtained from stems hardened in absolute alcohol ; J3, a partly diagrammatic 

 view of a vertical section of the wall, seen from within ; c, c, plan of section ; d. 

 opening to pit ; C, front view of young wall of a vessel ; , unthickened portion of 

 wall ; v, thickening-ridge ; Z, vertical section of C; E, section of wall m a place 

 where a vessel adjoins a succulent cell p; the thickening-ridges (g) are only on 

 one side. X 800. After Sachs. 



tionary," third edition, 1874; Carpenter's "The Microscope," fifth edi- 

 tion, 1874 



