DIV. I 



MORPHOLOGY 



65 



length. The widest as well as the longest vessels are met with in climbing plants ; 

 in them they may be 07 mm. wide, while those of the Oak are about 0'25 mm. and 

 of the Lime 0'06 mm. 



The terminology of the water-conducting elements is somewhat confused in 

 the literature. As a rule the distinction is drawn between tracheides and tracheae 

 or vessels. DE BARY, however, called all these elements tracheae and distinguished 

 between tracheides and vessels. The suggestion of ROTHERT which is adopted 

 here appears most convenient, viz. to distinguish within the collective conception 

 vessels, the tracheides and tracheae. 



The thickening of the walls of vessels may have the form of 

 narrow bars, T-shaped in cross-section (Fig. 68) on the relatively thin 



r* 



> 



FIG. 67. A, Part of an annular tracheide. B, 



;Part of a spiral tracheide. C, Longitudinal FIG. 68. Portion of a longitudinal section 



section through part of a reticulate trachea through three spiral vessels and a row of 



showing the remains of a partition wall, s. pareachyma cells of the Gourd (Cucurbita 



(x 240. After H. SCHEXCK.) Pepo). (x 560. After W. ROTHERT.) 



wall. These bars may form isolated rings, connected spirals, or 

 a network, and accordingly ANNULAR, SPIRAL, and RETICULATE 

 tracheides and tracheae are distinguished (Figs. 67, 68). In other 

 cases the thickening involves the greater part of the cell wall but 

 leaves numerous pits (PITTED VESSELS, Figs. 69, 70). The pits may 

 be circular, polygonal, or more or less transversely extended and 

 elliptical or slit-like. When transversely-extended pits stand above 

 one another in regular rows on the lateral walls the vessel is termed 

 SCALARIFORM (Fig. 69 //., 70 A). The pits of pitted vessels are 

 always BORDERED PiTs( 46 ), the canal of which widens from the cell 

 lumen to the pit membrane (Fig. 71). They may be present on one 

 or both sides of a cell wall. The outline of the pit in surface view is 

 commonly circular and encloses a smaller circle (Fig. 71 A). The 

 smaller circle is the opening into the cell cavity, while the wider 

 outline is that of the pit cavity at its widest part adjoining the pit 

 membrane. The thickening of the cell wall thus overhangs the 

 pit membrane and forms the wall of the pit, between the outer and 



