PINE STEM 248 



of the sections is the most accurately tangential, i.e. 

 that the plane of section is there most accurately perpen- 

 dicular to the radius of the stem. (See p. 8). Sections 

 should be cut at different depths in the tissues, so that 

 the middle of the plane of section shall traverse, (A) the 

 peripheral part of the xylem, (B) the cambium, and (C) 

 the inner part of the secondary phloem. Mount as 

 before. 



A. In sections which pass through the peripheral part 

 of the xylem observe 



i. The tracheides, of prosenchymatous form. No 

 bordered pits (or very few) are seen in surface view, but 

 they may be seen in large numbers in the radial walls 

 (here cut longitudinally), presenting a similar appearance 

 to that seen in transverse sections. (See p. 238). 



ii. Medullary rays, which resemble a section of 

 a biconvex lens. Note that each ray extends only a 

 short distance in a longitudinal direction : in some 

 cases rays consist of only a single radial series of 

 cells, of which only one lenticular cell appears in this 

 section. In those rays which consist of several rows of 

 cells, note the small triangular intercellular spaces 

 which intervene between the cells : these are only seen 

 in thin sections. Occasionally a resin-passage is in- 

 cluded in a ray. 



iii. Longitudinal resin-passages. 



B. In sections passing through the cambium will be 

 seen 



i. The cambium-cells, resembling the tracheides in 

 form (prosenchymatous) : the cell-walls are thin, and the 

 protoplasm granular, with elongated nucleus. 



ii. The cambium of medullary rays is similar in 



B 2 



