STUDY OF TRACIIEARY ELEMENTS IX SECONDARY XYLEM 89 



distinctly bordered pits whieli mark tiie point of contact between 

 the tracheid and the living cells of a wood-raj'. As seen in face 

 view, the large circular bordered pits of a spring tracheid are 

 separated from one another by bars of wall substance (cf. Eames 

 and MacDaniels, p. 29, Fig. 17A). These wall-sculpterings are 

 termed "Bars of Sanio" and have occasioned much speculation 

 as to their significance. The Committee on Nomenclature of the 

 International Association of Wood Anatomists, however, sug- 

 gests that the term "Bars of Sanio" should be replaced by the 

 term "crassulae" which are defined as "thicker portions of the 

 intercellular layer and primary walls between primary pit 

 fields." To appreciate fully the form and distribution of pits in 

 conifer tracheids, a study should also be made of stained trans- 

 verse, radial and tangential sections of pine wood. 



2. Traclieids of angiosperms. INIount a small quantity of 

 macerated oak wood in water and study carefully the form and 

 pitting of the tracheids. These cells are distinguished from the 

 very abundant wood fibers by their somewhat shorter length, 

 wider lumina and more obviously bordered pits. Many of the 

 tracheids will appear very irregular in contour with forked or 

 lobed ends. (Cf. Eames and MacDaniels, p. 60, Fig. 33D). 

 Under high magnification, note that the bordered pits are oval 

 in form, often crowded and somewhat smaller than the larger 

 circular bordered pits characteristic of the spring tracheids of 

 Pin us. 



3. Vessel elements with sealariform perforations. Macerated 

 wood of birch (Betula) and of the tulip-tree {TAriocUndron) will 

 provide instructive examples of this primitive type of vessel 

 element. Note the variation in the number and relative width 

 of the slit-like openings in the oblique perforation plates of these 

 two plants, and the variety in type and arrangement of pits on 

 the lateral walls. 



4. Vessel elements with simple perforaiions. As stated in the 

 Introduction of this exercise, the simple type of perforation may 

 occur in vessel elements with either oblique or more or less trans- 

 verse end-walls. The first condition is shown by certain of the 

 vessel elements in macerated oak wood. Note in this material that 

 the pointed ends of the vessel element extend beyond each of the 



