144 PRACTICAL BOTANY. 



which is always present, traversing the pit-cavity in all 

 cleanly cut sections; the pits are therefore not per- 

 forated. 



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 xylem-paren- 

 chyma. 



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- 

 tial extension. This is necessary to keep pace with the 

 increase in bulk of the vascular system. 



2. The phloem is thicker, and the constituents of the 

 outer part of it are much distorted and displaced. 



3. The xylem has increased, in thickness more than 

 any other tissue, so that it is now the chief constituent 

 of the stem. It may be distinguished as being com- 

 posed of three bands (annual rings), in each of which 

 the more central tracheides have large cavity and thinner 

 walls (wood developed in spring) ; passing outwards 

 there may be seen a gradual reduction of the cavity, and 

 increase in thickness of the walls till a certain limit is 

 reached (autumn wood). Outside the latter is a sudden 



