al 
64 RLEMENTS OF PLANT ANATOMY. 
otherwise interrupt its growth, and by this means it retains and 
enhances the peculiarity of a sharpened end. 
Bast cells usually have numerous pores which are, however, 
seldom circular but more often oval or oblong. Their long axis 
is generally oblique to that of the cell and inclined to the left. 
It is believed there is some relation between this position and 
the arrangement of the micellae in the striation of the wall. 
Circular pores do sometimes occur and also oblong pores with 
their long axis towards the right, but these are not frequent, and 
may be regarded as exceptional. 
ELEMENTS OF THE XYLEM. 
The ducts or tracheae are the leading element of the xylem 
and correspond in this respect to the sieve-tubes of the phloem. 
They were named tracheae when their chief function was sup- 
posed to be that of air passages. The name vessel was also 
given them from the fact that they had lost the characteristic 
mark of cellular tissue, which is that it consists of closed cells. 
From this name comes the word vascular, which is still used to 
characterize the entire class of elements derived from the cam- 
bium. Like all other prosenchymatic elements cut off from the 
cambium, they are at first long, narrow, prismatic cells. They 
afterward increase not only in length but also in the other two 
dimensions and not infrequently become nearly cylindrical. 
During the course of this growth the transverse walls become 
either entirely absorbed, or perforated by openings of various 
shapes, so that a direct communication exists between the super- 
imposed cells. Another important feature in their development 
is the manner of growth in thickness of the remaining walls. 
This, instead of being uniform like that just described of the bast 
cells, is interrupted in various ways, leaving thin places, which 
allow the easy passage of fluids, while the thickened portions 
furnish the necessary support. Such markings of wall have 
