FIBROVASCULAR TISSUES: TRACHEIDS AND FIBERS 31 



bands of tracheids in intimate relation to the vascular ele- 

 ments which they surround. Laterally to the stripes of 

 tracheids in a general radial direction lie other denser areas 

 made up of mechanical elements known as fiber-tracheids. 

 There is scarcely any marked boundary between the fiber- 

 tracheid and the tracheid proper, although 

 the extreme conditions of both are distinct. 

 Fig. 24 represents these two kinds of elements 

 as seen in longitudinal aspect in a maceration 

 of the wood. The tracheid is much shorter 

 and broader, has thinner walls, and is abun- 

 dantly pitted, both radially and tangentially, 

 with pores provided with oblique mouths. 

 In the case of the fiber-tracheids we have to 

 do with elements of considerable length and 

 narrow lumen, inclosed by rather thick walls. 

 The pits are much more scanty, but are 

 equally distributed, as in the case of tracheids 

 proper, on both radial and tangential walls. 

 A marked feature of the pits in these ele- 

 ments is the extreme length of their oblique 

 mouths, which much exceeds the diameter 

 of the pit membrane. This situation becomes 

 the rule in tracheary elements devoted more to 

 the mechanical than to the water-conducting 

 function. 



In the mass of woods of the dicotyledons 

 the occurrence of tracheids proper and ex- 

 tremely differentiated mechanical elements 

 side by side with gradual transitions is not 

 seen. Usually the elongated elements of the 

 wood other than vessels (to be discussed in a 

 later chapter) are of a more or less definite 

 mechanical nature and have, as a conse- 

 quence, lost more or less completely the characteristics of 

 tracheids. Fig. 25 illustrates several types of fiber- 

 tracheids and similar structures. In a is seen a tracheid 



FIG. 24. Tra- 

 cheids and fiber- 

 tracheids of the 

 oak. 



