138 PRACTICAL BOTANY 



was cut in winter the transition will appear sudden, if cut in 

 summer it will appear gradual. 



Observe that the xylem-ring is cut by the medullary rays into 

 wedge-shaped areas. The chief tissue-elements filling these areas 

 are the tracheides, which present the following characters : 



i. They have approximately the same shape as the cells of 

 the cambium from which they are derived. 



ii. Their walls are thick and lignified (yellow with chlor-zinc- 

 iodine), and are differentiated into layers, distinguished optically 

 and by staining. 



iii. They have no cell-contents. 



iv. On their radial walls (and rarely on the tangential walls) 

 are found the bordered pits, which are best seen in the xylem 

 formed at the early part of the year. 



These appear, when seen in transverse section under a low 

 power, as bi-convex enlargements of the wall, which look 

 darker than the rest of the wall : under a high power it is seen 

 that there is a biconvex-lens-shaped cavity of the pit, over- 

 arched on either side by a meniscus-like outgrowth of the wall, 

 which is seen in good sections to be perforated at its centre : 

 the cavity of the pit is traversed by the thin pit-membrane, 

 enlarged at its central point, -and occupying either a median 

 position, or more commonly arched to one side : the wall is 

 thus not perforated at the pits, but they are due to irregularity 

 of thickening of the wall, the thinnest part remaining persistent 

 as the pit-membrane. 



Observe near the centre, and bordering on the pith, the 

 protoxylem arranged as above observed in the younger stem. 

 No bordered pits occur in the walls of the protoxylem. 



Note the occurrence of resin-passages in the secondary 

 xylem, lined as before by thin-walled epithelium which may be 

 regarded as a form of xylem-parenchyma. 



II. Cut transverse sections of a three-year-old-stem so as to 

 include the whole width of the vascular ring ; it is not necessary 

 however to have a complete transverse section of the whole 

 stem. Mount in glycerine. Comparing this -with what has 

 already been observed in the stem of the current year, note the 

 following differences : 



