802 THE AMERICAN NATURALIST. (VoL. XXXIX. 
have distinctly approached that modification which must imme- 
diately precede the formation of bordered pits. Such a feature 
serves to show that the transition from spiral to scalariform 
structure might be accomplished without any very conspicuous 
transitional forms such as have been shown to exist in the case 
of Cordaites, and such as are found not to exist in the Salicaceze 
as a rule; while it also serves to explain in part the great radial 
diminution of the protoxylem of the Salicaceze as compared 
with that of more primitive forms of plants. In addition to the 
spiral tracheids, the protoxylem also includes a certain num- 
ber — generally few — of scalariform elements. These approxi- 
mate in form and size to the spiral tracheids though they 
often have distinctly tapering extremities, in which respect they 
approximate very closely to the fibrous mechanical elements 
or wood cells of the secondary xylem (Fig. 1). It 
has also been observed in such cases, that the 
scalariform markings become distinctly fewer and 
more distant (Fig. ı) as if certain of the pits had 
been obliterated by a more general secondary 
thickening of the wall, and that such is the true 
explanation we are justified in believing from a 
study of the transition zone in Cordaites (Penhal- 
low, :00, :04c). That such obliteration has not 
proceeded at a uniform rate throughout the length 
of the cell, is evident from the figure given. When 
such scalariform markings are exposed in section, 
they exhibit precisely the same bordered structure 
mp m in the case of the spirals. The most interior 
waursi. Ta. Portion of the secondary xylem sometimes contains 
pep cuam  SCalariform elements of a transitional form. In 
tracheid show. Such cases the scalariform structure is irregular 
Ma Iu and very imperfectly developed upon both the 
radial and the tangential walls, presenting an 
obvious form of transition in the direction of the bordered pits, 
chiefly expressed in the shortening of the individual pits and 
their more definite segregation. But such changes in the 
Structure of the cell wall might equally well be precedent to 
the complete and final obliteration of the pits of whatever form, 

