66 ELEMENTS OF PLANT ANATOMY. 
In some cases the cross walls are completely absorbed, in 
others a slight edge only is left. Others, again, retain the wall, 
but it is either crossed by oblong openings, like a gridiron, or 
contains a single large central opening. In all cases the fully 
developed duct is no longer a living element, its protoplasmic 
contents being replaced by air or water, or as is more frequently 
the case, by alternating columns of both. In some woods the 
duets are filled with resinous products, secretions of tannin and 
milky juices. There is also a pecuhar growth found in the 
cavities of certain ducts to which has been given the name of 
tylose. It occurs where a thin-walled parenchymatic cell lies 
adjacent to (or borders on) the duct. At a thin place in the 
latter, the parenchymatic cell pushes its way into the cavity of 
the duct and grows out into a large sac, which may afterwards 
be separated from its mother cell by a wall and so entirely 
enclosed in the duct. It either remains in this condition, or 
divides into numerous cells and continues growing till large 
masses are formed. These tyloses have been found to be nor- 
mal appearances in porous ducts of certain woods. In others, 
they seem to result from outward injury in such a manner as to 
indicate a pathological origin. 
TRACHEIDS. 
Tracheids are to the ducts what the accompanying cells are 
to the sieve-tubes of the phloem, namely assisting cells. They 
differ from the ducts as the accompanying cells differ from the 
sieve-tubes, by having no immediate connection with each other. 
In other words they are closed cells whose transverse walls are 
neither removed nor perforated by openings; their contents 
therefore can pass from one to the other only by osmosis or 
filtration. In shape and in markings of wall they resemble the 
ducts closely, but their average size is considerably less. It is 
often difficult to distinguish them from small ducts, as the only 
