TRACHEARY TISSUE 37 
thin places remaining only as small pits. The cells of all 
these structures are usually more or less pointed and over- 
lapping at the ends, except in some of the tracheae in 
which the square end walls were dissolved out. They 
are mostly round or by mutual pressure somewhat angled 
in cross-section. 
48. The spiral and annular thickenings are the 
only types found in the tracheary tissue that is formed in 
stems or roots that are still elongating, as it is possible 
for such cells to elongate by the stretching or growth of 
the unthickened portion, whereby the rings become 
farther apart or the spirals stretched out at a greater 
angle. Very often adjacent rings may be connected here 
and there by a spiral or the same vessel may have annular 
thickenings in one part and spiral in another. There 
may be from one to three or four spirals. The reticu- 
late type of thickening is perhaps to be considered as a 
many-spiraled type with numerous cross connections 
from one spiral to the next so as to form a network. 
Scalariform vessels are usually angular in cross-section 
and have their thickenings on the flat faces of the prisms 
as horizontal bars connected to the somewhat thickened 
angles, and leaving horizontally elongated thin areas be- 
tween them like the openings between the rungs of a 
ladder. All transitions may be found from the reticu- 
lated or scalariform structure to the pitted type. The 
pitted tissues are of two types: (a) with simple pits, and 
(b) with bordered pits. In the first the pits are of the 
same diameter through their whole depth or even wider 
toward the center of the cell. In the second, they are 
narrow, adjacent to the cell lumen and are much wider as 
they approach the middle of the cell wall, the cavity of 
each pit having the shape of a planoconvex lens. The 
wall or diaphragm separating the adjacent pits of ad- 
