240 PRACTICAL BOTANY 



2. The xylem, consisting of 



a. Tracheides with lignified walls, and no cell-con- 

 tents. Starting from those nearest the pith, and 

 passing outwards, the following forms may be observed, 

 and distinguished mainly by the markings due to un- 

 equal thickening of the walls. 



i. Tracheides with narrow cavity, and more or less 

 regular annular or spiral marking the proto- 

 xylem. 

 ii. Elements wider than these, and with bordered 



pits scattered between the spirals, 

 iii. Normal tracheides, with bordered pits only : 

 these form by far the greater bulk of the 

 secondary xylem, and must be carefully 

 studied. Their form is prosenchymatous. 

 The greater part of the cell- walls is of uniform 

 thickness. On these portions of the wall 

 observe with the high power two intersecting 

 systems of lines of striation. In single 

 longitudinal rows are found the bordered 

 pits : each of these has the appearance of 

 two concentric rings, of which the smaller is 

 more strongly marked, and corresponds to the 

 opening of the cavity of the pit into the cell- 

 cavity, the larger represents the limit of area 

 of the pit. It must be remembered that we 

 are now observing the radial walls in surface 

 view. A careful comparison should be made 

 of the bordered pit as seen here in surface 

 view with its appearance when seen cut 

 through, as in the transverse sections above 

 described. 



