FORMS OF CELLS. 
45 
horizontal or oblique rings are known as annular 
ducts; those having the thickenings in the form of 
spirals, which usually run from right to left, are 
known as spiral ducts; those having the thickenings 
in the form of a reticulation are known as reticulated 
ducts, and those with horizontal, disconnected thicken- 
ings which occur in parallel lines, resembling a flight 
of steps, are known as scalariform ducts. 
In those ducts in which but few of the transverse 
walls are obliterated, the walls aie marked by both 
simple and bordered pores (for a description of the 
latter, see Tracheids). 
Ducts contain water, water-vapor and air; in some 
cases they contain sugar, tannin, mucilage or resin. 
The tracheids are intermediate in character between 
ducts and wood fibers, resembling the former in pos- 
sessing bordered pores 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 phloroglucin 
and aniline sulphate. 
One of the chief characteristics of tracheids are the 
bordered pores. These differ from simple pores in 
that the wall surrounding the pore forms a dome- 
shaped or blister-like 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. 
The number and distribution of bordered pores in 
the Coniferse are quite characteristic for some of the 
genera, and may be studied in any of the pines, the 
pores being most numerous along the radial walls. 
The sieve (sieve tubes) is distinguished from the other 
conducting elements, in that the walls are thin and 
