146 PRACTICAL BOTANY. 



cell-walls is of uniform thickness. On these por- 

 tions of the wall observe with the highest power 

 two intersecting systems of lines of striation. In 

 single longitudinal rows are found the Bordered pits ; 



each of these appears as two concentric rings, of which 

 the smaller is more strongly marked, and corresponds 

 to the opening of the pit into the cell-cavity. 



It must be remembered that we are now observing the radial 

 walls in surface view. Compare the bordered pit as seen here 

 with its appearance when seen in transverse section. 



Note the annual rings recognised here, as in the 

 transverse sections, by difference in width of cavity, and 

 thickness of walls of the tracheides of the .xylem. 



5. Here and there the continuity of the mass of 

 tracheides is broken by a longitudinal resin-passage, 

 surrounded by parenchymatous cells (xylem-paren- 

 chyma), which have cellulose walls and retain their 

 cell-contents. 



The whole mass of xylem is traversed radially by 

 plates of parenchyma (Medullary rays). Note that 

 they extend only a short way longitudinally, but a long 

 way radially ; also that they are composed of cells 

 arranged like bricks in a wall, among which may be 

 be distinguished 



a. Cells with cellulose walls, and protoplasmic 

 contents. The walls of the tracheides which abut on 

 these are unusually wide. 



b. Elements, with no protoplasm, and with lignified 

 walls marked with bordered pits. 



Both tissue-forms may often be found in the same 

 ray, though rays will often bo seen consisting of 

 (b) alone. 



