r 4 o PRACTICAL BOTANY 



must be carefully studied. Their form is prosenchy- 

 matous. 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. The bordered pits are found in single longi- 

 tudinal rows: 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. 



Note the annual rings recognized here as in the transverse 

 sections, the autumnal wood being distinguished by 

 the smaller size of the cavity and greater thickness of 

 the walls of the tracheides. 



b. Here and there the continuity of the mass of tracheides is 

 broken by a longitudinal resin-passage, surrounded by 

 parenchymatous cells, which have cellulose walls and retain 

 their cell-contents. 



c. 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 distinguished 



i. Cells with cellulose walls, and protoplasmic contents : 

 the pits in the walls of the tracheides which abut on 

 these are unusually wide, 

 ii. 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 be seen consisting of (ii.) alone. Note that 

 between the cells (i.) clearly defined lines running radially may 

 commonly be seen : these are intercellular spaces. 



3. The cambium-layer, consisting of elongated thin- walled 



