THE AMERICAN XA TURALIST. [Vol. XXXV I II. 



species, while it is often of distinctly greater value because 

 affording data derived from presumably more primitive types. 

 Thus such transitions are well known, though of a relatively sim- 

 plified form in the structure of the calamitean stem, and an 

 excellent example of this kind is afforded by a figure given by 

 Scott (43, p. 22). In 1869 Williamson (52, p. 69) directed 

 attention to the occurrence of such transformations in the tra- 

 cheids of Cordaites (Dadoxylon). A more recent study of this 

 genus (39, p. 57 and 43, p. 418) has shown that this feature is 

 well exhibited in C. brandlingii, where a suitable radial section 

 (Figs. 1-4) will present a more or less graduated series of transi- 

 tions from the typical spiral tracheid of the protoxylem, through 

 scalariform structures to the multiseriate bordered pits of the 

 tracheids in the secondary xylem ; while within the limits of the 

 same tracheid, such transitions may be observed as it were, in 

 process of development, affording the most conclusive proof in 

 this respect. These transitions as observed in Cordaites brand- 

 lingii show the following phases : 



In the successive radial development of new tracheids, there 

 is a constant tendency to a more uniform thickening of the cell 

 wall by secondary growth. This at first finds expression in the 

 more compact arrangement of the spirals which later coalesce at 

 various points, thus giving rise to more localized areas devoid 

 of secondary growth, and hence to a scalariform structure in 

 which the general lines conform more or less exactly, to the 

 direction of the original spirals. By a further modification the 

 elongated, thin areas become converted into shorter, often isodi- 

 ametric areas substantially by a process of division. A further 

 tendency to general thickening of the walls causes the margins 

 of the scalariform structure to project from all sides and extend 

 over the area of arrested growth as a lip which never completely 

 closes at the centre, where there is left a usually circular, some- 

 times oval or again lenticular or even oblong opening, and in this 

 manner the bordered pit is formed. 



The region within which these changes occur, or the " transi- 

 ti.>n zone," is subject to great variation whereby the change from 

 spiral to bordered pit may arise very gradually through a broad 

 radial zone, as in Cordaites brandlingii or it may occur very 



