PROTOXYLEM AND METAXYLEM 295 



parenchymatous rays which are continuous with the' 

 pericycle. In slender cyHnders, where the number of 

 alternating strands of xylem and phloem is small the 

 centre of the cylinder is filled up with large xylem 

 vessels in contact with the strands of xylem which 

 abut on the pericycle (Fig. 48). In massive cylinders 

 where the number of xylem and phloem strands is 

 large the centre is occupied by parenchyma {pith). 



The outermost tracheids, abutting on the pericycle, 

 are narrow, and spiral or annular. They are the first 

 xylem elements to be formed, and are hence called 

 protoxylem (Fig. 48, px.). Within these are larger pitted 

 vessels {metaxylem) (Fig. 48, mx.), which, in a narrow 

 cylinder, fill up the centre (Fig. 48). The spiral or 

 annular thickenings of the outermost tracheids are 

 laid down and lignified in the elongating region of the 

 root, the protoplasm dying after the thickenings are 

 complete. The thin cellulose wall of the tracheid is 

 passively stretched by the active elongation of the 

 surrounding parenchymatous tissue, and the successive 

 turns of the spiral thickening (or the annuli) are pulled 

 apart (Fig. 45, B — G). As long as they are not too 

 widely separated, the internal thickenings of the tracheid 

 prevent the obliteration of the cavity by the pressure 

 of the surrounding turgid tissue, and so this first 

 formed water-conducting channel is kept open. 



Thus the spiral and annular tracheids are very 

 beautifully adapted to perform their function under 

 the conditions in which they are formed, for they both 

 admit of longitudinal extension to keep pace with 

 the growth in length of the tissue, and are protected 

 against obliteration by the surrounding turgid tissue. 

 In organs which elongate a great deal, the spiral 

 tracheids first formed are ultimately stretched so much 



