ANATOMY OF TISSUES. 67 
difference is that while the cross walls of the ducts are perforated, 
those of the tracheids are entire. In many instances, however, 
they resemble the libriform cells in size, length, and pointed 
ends, and in the tendency to thicker walls than those of the 
ducts. Formerly the ducts and tracheids were classed together 
as a single element ; now the tendency is not only to consider 
them as distinct from each other, but to class the tracheids with 
the libriform tissue rather than with the ducts. Without 
question there are transition forms all the way between libriform 
cells on the one hand, and ducts on the other, and these forms 
pass by such insensible gradations into each other that it is dif- 
ficult to say where the exact limit is. 
LIBRIFORM TISSUE. 
This name was first given by Sanio to that element of the 
xylem which corresponds to the bast of the phloem. There is 
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FIG. 33. 
Radial long section through wood of oak. g ducts. hp wood parenchyma, t tracheids. 
llibriform. x 300. — Wiesner. 
little difference between bast and libriform tissue except that of 
location, but the degree of lignification in the walls of the latter 
is apt to be greater than in bast. In some lbriform tissue of 
