142 PRACTICAL BOTANY 



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. 



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 included in a ray. 



iii. Longitudinal resin-passages. 



(ff) 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 shape to the 

 cells of the rays : it is thin-walled, with granular protoplasm 

 and nucleus. (Compare Fig. 7, C, p. 64.) 



If these sections be treated with dilute potash, and mounted 

 in glycerine, their structure may be more easily made out. 



(c) In sections passing through the phloem will be seen 



i. The medullary rays as before, but their form is more 

 convex : all the tissues between the medullary rays are de- 

 rived from cambium-cells of the form above observed. These 

 are 



ii. Sieve-tubes, which retain the form of the cambium-cells : 

 the cellulose walls seen in surface view are smooth : those cut 

 longitudinally appear of wavy outline (sieves) : the structure of 

 the latter is well seen after treatment with chlor-zinc-iodine for 

 twenty-four hours. 



iii. Bast-parenchyma, derived from cambium-cells by their 

 division by transverse walls. 



iv. Some few cells, especially towards the periphery con- 

 taining crystals which give the reactions of calcium oxalate. 



