486 BOTANICAL GAZETTE [DECEMBER 
the “typical tracheid” or “true tracheid.” This form, which occurs 
in the wood of practically all of the broadleaf trees, is analogous 
to the tracheid of the conifers. From the primitive tracheid form 
there seem to have developed throughout the broadleaf tree species 
two highly specialized forms, vessel and wood fiber. In the genus 
Platanus the general term “‘tracheid”’ must be made to include all 
transitional forms between the typical tracheid and the vessel on 
the one side, and between the typical tracheid and the wood fiber 
on the other side. 
The typical tracheid is moderately thin-walled, has oblique 
simple pits, and the perforations at the ends are slitlike (fig. 1, £). 
The tracheid forms between the typical tracheid and the vessel 
possess, in addition to the oblique simple pits, rows of oblique 
bordered pits and transverse simple pits, both of which forms occur 
in the walls of vessels (fig. 1, A and D, bp and sp); and the perfora- 
tions at the ends may be simple, either with or without bars, or 
scalariform, as in vessels; or the perforation may be a transitional 
form between the scalariform as found in vessels and the slitlike 
perforations found in true tracheids (fig. 1, D, sp). The tracheid 
forms between the typical tracheid and the wood fiber are some- 
what slender, pointed at both ends, and thick-walled, and possess 
the vertical bordered pits of wood fibers in addition to the oblique 
simple pits belonging to tracheids. They often have also small 
transverse slits like those in the ends of true tracheids (fig. 2, D, 
slp). These tracheid forms (or tracheids) of sycamore wood, 
therefore, although extremely variable, may be defined as moder- 
ately thin-walled, elongated elements with slightly oblique ellipti- 
cal or slitlike simple pits and slitlike perforations at the ends. 
They may also possess those pits common to either vessels or 
wood fibers, and the perforation at the ends may be simple, 
scalariform, or slitlike, these types often grading into each other. 
The average tracheid is 1.3 mm. in length and about 0.04 mm. In 
diameter. 
3 A study of such transitional forms as are found in the wood of the sycamores is 
of great value to the student in wood structure, in that it shows the relationship of 
the elements to each other and assists in their classification and in the recognition of 
the essential features belonging to each class. 
