136 PHARMACEUTICAL BOTANY 
found, each opening into a different cell. ‘The cross or end wall 
separating one trachea from another usually possesses a single, 
large, rounded or elliptical perforation called a simple perforation, 
but some end walls possess two or more smaller perforations 
forming an opening divided by transverse bars and called a 
scalariform perforation. ‘The longitudinal or vertical walls of these 
tubes are of varying thickness, usually, however, thinner than 
those of woody fibers. ‘The thickness is due to an infiltration of 
lignin upon the original cellulose wall. The walls show char- 
acteristic thickenings on their inner surfaces. These thickenings 
are for the purpose of giving strength to the tube while the thin 
parts of the wall are for the passage into and out of the tube of 
water with nutrient materials in solution. 
TRACHE/# are Classified according to their markings as follows: 
Annular, with ring-like thickenings. 
Spiral, with spiral thickenings. 
Reticulate, with reticulate thickenings. 
Porous or pitted, with spherical or oblique slit pores. 
Bordered pored, with rows of pores or slits, each being sur- 
rounded by a thickening of variable shape (circular, ellipsoidal 
- and prismatic are the usual shapes). 
Annulo-spiral, with both ring and spiral thickenings. 
Scalariform, with ladder-like thickenings. 
TRACHEIDS are primitive conducting xylem cells which have 
thickenings similar to trachea but are usually, though not always, 
smaller in size. Their ends are usually tapered, sometimes 
chisel-like, but not sharply pointed. They may be distinguished 
from trachez: by the absence of perforations in their end walls. 
Their end walls have, however, thin places or pits which permit 
the more rapid passage of sap from cell to cell. Like trachee 
their walls usually give the characteristic lignin reaction with 
phloroglucin and HCl. The tracheids of conifers (see Fig. 75E, 
D.) such as the pine, spruce, cedar, etc., usually show one row 
of good-sized bordered pores (bordered pits) on their longitudinal 
walls. Each bordered pore exhibits a wall surrounding the pore 
which forms a dome-shaped protrusion into the cell. Like 
trachez, also, tracheids convey water with mineral salts in solu- 
tion upward through the plant axis. Tracheids with bordered 
