238 PEACTICAL BOTANY 



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 epithe- 

 lium, which may be regarded as a form of xylem- 

 parenchyma. 



III. Cut transverse sections of a three-years-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 : 



1. The cortical tissue bears evident traces of tangen- 



