68o 



ECOLOGY 



the bundle termini of angiosperm leaves, tracheids usually remain as 

 such, that is, the end walls are not resorbed (fig. 1003). 



Tracheids are much shorter than tracheae, rarely exceeding a millimeter in length, 

 though sometimes attaining a length of twelve centimeters (as in Nelumbo) or even 

 a meter (as in some conifers). ' Tracheae rarely are longer than ten centimeters, 

 though they may attain a length of one or two meters in Quercus and three to six 

 meters in lianas (as Wistaria) and in Eucalyptus. While isolated tracheids and 

 tracheae sometimes occur, they usually are grouped in continuous strands traversing 

 the entire plant body. 



Individual tracheids or tracheae, even within a single conductive strand, vary 

 widely in wall sculpturing, owing to differential lignification. When the thickenings 



m 



FIG. 1002. A diagrammatic longitudinal section 

 of a young xylem strand; c, cambium: -y, a young 

 undifferentiated trachea with cross walls as yet un- 

 resorbed; p, a trachea with transversely elongated 

 pits; s, spiral tracheae; o, an annular trachea; 

 m, pith; highly magnified. From BARNES (Part 

 II). 



FIG. 1003. Tracheids of 

 a gymnosperm with bordered 

 pits; highly magnified. Alter 

 CHAMBERLAIN. 



are annular or spiral, the wall area largely remains thin (figs. 1002, 1004, 1005). 

 In reticulated vessels, the strengthening fibers form a network, and in the pitted tra- 

 cheids and tracheae, most of the wall becomes thickened, the thin places appearing as 

 small pits (fig. 1006). Pitted and reticulated vessels usually are larger than those 

 with annular or spiral thickenings. . Still another form of thickening is seen in the 

 scalariform vessels, in which transversely elongated pits occur in parallel rows (fig. 

 1000). Bordered pits occur in many plants, especially in the conifers, where they are 

 arranged in longitudinal rows, a surface view disclosing two concentric rings or el- 

 lipses (fig. 1003) ; in section there are seen a thickened central portion, the torus, and 



