TRACHEARY TISSUE 37 



thin places remaining only as small pits. The cells of all 

 these structures are usually more or less pointed and over- 

 lapping at the ends, except in some of the tracheae in 

 which the square end walls were dissolved out. They 

 are mostly round or by mutual pressure somewhat angled 

 in cross-section. 



48. The spiral and annular thickenings are the 

 only types found in the tracheary tissue that is formed in 

 stems or roots that are still elongating, as it is possible 

 for such cells to elongate by the stretching or growth of 

 the unthickened portion, whereby the rings become 

 farther apart or the spirals stretched out at a greater 

 angle. Very often adjacent rings may be connected here 

 and there by a spiral or the same vessel may have annular 

 thickenings in one part and spiral in another. There 

 may be from one to three or four spirals. The reticu- 

 late type of thickening is perhaps to be considered as a 

 many-spiraled type with numerous cross connections 

 from one spiral to the next so as to form a network. 

 Scalariform vessels are usually angular in cross-section 

 and have their thickenings on the fiat faces of the prisms 

 as horizontal bars connected to the somewhat thickened 

 angles, and leaving horizontally elongated thin areas be- 

 tween them like the openings between the rungs of a 

 ladder. All transitions may be found from the reticu- 

 lated or scalariform structure to the pitted type. The 

 pitted tissues are of two types : (a) with simple pits, and 

 {b) with bordered pits. In the first the pits are of the 

 same diameter through their whole depth or even wider 

 toward the center of the cell. In the second, they are 

 narrow, adjacent to the cell lumen and are much wider as 

 they approach the middle of the cell wall, the cavity of 

 each pit having the shape of a planoconvex lens. The 

 wall or diaphragm separating the adjacent pits of ad- 



