THE PRIMARY PERMANENT TISSUES 39 



While a tracheal tube consists of a chain of fused cells a tra- 

 cheid is a single cell only with thin places in its walls in the form 

 of pits with overhanging border for greater strength. These are 

 known as bordered pits (Fig. 19). Or the thickenings may be of 

 the spiral and reticulate types. In becoming a tracheid the 

 procambium cell elongates and tapers at the end to a greater or 

 less degree, thickens its wall unequally, and finally lignifies its 

 wall (Fig. 19). 



The tracheal tubes are primarily for carrying water from roots 

 to and throughout the leaves; and the tracheids have the same 

 function, but they may assume the character of wood fibers and 

 be depended on for strength as well as for conducting water, as 

 in the case of pine wood. 



The cells of the xylem parenchyma are, as a rule, relatively 

 thin-walled, and the walls are sometimes, and in woody plants 

 commonly, Hgnified, and they may or may not be pitted. To 

 form the xylem parenchyma the procambium cells divide trans- 

 versely, and the young parenchyma cells enlarge in all dimen- 

 sions, becoming more or less elongated vertically, with end walls 

 at right angles to the vertical or, as a rule, only slightly inclined 

 (Fig. 19). They differ from the tracheids by their less elongation, 

 blunter ends, commonly thinner walls and unbordered pits. 

 The proportion of xylem parenchyma to the other elements of 

 the primary xylem varies greatly in different examples, from 

 occupying the bulk of the xylem to entire absence. Its function 

 is to store reserve water and foods, and possibly to assist in lifting 

 the water to the leaves. 



The wood fibers are characterized by being much elongated 

 and taper-pointed at the ends, and by thick and lignified walls 

 and small unbordered pits. The steps in the evolution of a 

 wood fiber from a procambium cell are evident. The cell elon- 

 gates, and in doing this the ends of contiguous cells shove past 

 each other; the walls gradually thicken, and finally become^more 

 or less ■ completely Kgnified. The wood fibers are chiefly to give 

 strength, and they are assisted in this by their interlacing and 

 dove-tailing together, as well by their thick and lignified walls. 



