PITTED TRACHEIDS 45 



have been wholly obliterated, -- which had its origin at a point 

 anterior even to such types as Cordaites, and therefore in a group 

 probably represented by the Cycadofilices. 



PITTED TRACHEIDS 



The pitted tracJieids may be so distinguished because of the 

 invariable presence of bordered pits upon their radial and, under 

 some circumstances also, upon their tangential walls. Such pits 

 belong to the secondary layer of the cell wall in all cases. In 

 comparatively few instances such pits may be associated with 

 spirals in the tertiary wall, when the two structures will be found 

 so related that while the latter may overlap the former, the 

 orifice always lies between the turns of the spirals (figs. I and 2). 



In their transverse aspect there is no feature which may be 

 employed to distinguish the pitted from the spiral tracheids. In 

 the longitudinal aspect the former differs from the more primi- 

 tive forms of the spiral tracheid with respect to length and 

 definite terminations in such way that the one is a wood cell 

 while the other is a vessel, but between the two no sharp line 

 of demarcation can be drawn, since, as already indicated, they 

 pass the one into the other by insensible gradations. 



All tracheids exhibit the same structural features with respect 

 to the development and composition of the wall, and as a knowl- 

 edge of these is antecedent to a correct understanding of certain 

 alterations which arise incidentally to growth and also to decay, 

 it will be desirable to examine into the structure of the wall 

 somewhat in detail. 



Assuming the primitive form of the cell to be that of a sphere, 

 this form undergoes alteration in accordance with the immediate 

 environment whereby growth first of all becomes excessive in 

 one direction coincident with the axis of growth for the plant 

 as a whole, while it remains practically equal in the radial and 

 tangential planes which cut the first at right angles. From this 

 it follows that while in a longitudinal aspect the tracheid is 

 always presented as a fibrous element with tapering extremities, 



