PINE STEM 241 



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 ligHified 



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.) there may 

 commonly be seen clearly defined lines running radially : 

 these are intercellular spaces. (See below p. 243). 



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

 walled cells, the ends of which are difficult to observe. 

 They have copious protoplasm, and an elongated nucleus. 

 (Compare Fig. 9, B, p. 107). 



Note that the medullary rays are continuous through 

 the cambium, and observe the differentiation from 



K 



