CELLS AND TISSUES -l-j 



variously thickened and reinforced, are abundantly pitted, espe- 

 cially at the ends where the cells interlock with one another. The 

 pits are bordered, although in some cases the borders are so narrow 

 that the pits appear to be simple. Thus a vertical series of tra- 

 cheids forms a conducting strand through which water may move 

 freely. 



The character of the secondary wall determines the type of the 

 conducting element. In addition to fitted forms, annular, spiral, 

 scalariform, and reticulate elements may be differentiated in the 

 primary xylem as a result of the deposition of secondary wall mate- 

 rials in various patterns. (Fig. 8.) The annular and spiral types 

 are most commonly found in the protoxylem. In the former, there 

 is a series of separate ring-like reinforcements, the spacing between 

 adjacent rings depending in part upon the rate and degree of matur- 

 ity of the element, the rings being farthest apart in the most rapidly 

 growing annular elements because of the vertical elongation of the 

 primary wall between them. In the spiral element, the bands of 

 the secondary wall are arranged in a compact or loosely coiled 

 spiral, the latter condition occurring in protoxylem elements which 

 are differentiated before the elongation of the axis has been com- 

 pleted. It is not uncommon to find transitional forms between 

 these two types in which both spiral and annular thickenings occur. 



The scalariform and reticulate elements usually develop in the 

 metaxylem. In the former, the secondary thickenings are laid 

 down in a close series in a ladder-like pattern. The reticulate 

 element has more secondary wall thickening than the scalariform, 

 and the mesh of the network is much finer. Because the proportion 

 of secondary wall material to total wall area is much greater, the 

 scalariform and reticulate elements are more rigid than the annular 

 and spiral types. This affords a good example of structural and 

 functional correlation, since the annular and spiral elements of 

 the protoxylem are commonly differentiated before complete 

 elongation of the axis is accomplished; while the more rigid 

 scalariform and reticulate elements of the metaxylem are usually 

 formed after this has occurred. 



The Vessels. — The vessel consists of a vertical series of cells 

 (vessel segments') in which the adjacent end walls are perforated 

 in various ways so that a continuous tube is formed. Frost (lo, 

 ii), in discussing the origin of the vessel, points out that it is 

 clearly "a series of individual cells, properly called vessel elements 



