PINE STEM 141 



cells, the ends of which are difficult to observe. They have 

 copious protoplasm, and an elongated nucleus. (Compare 

 Fig. 7, B, p. 64.) 



Note that the medullary rays are continuous through the 

 cambium, and observe the differentiation from the uniform 

 cambium of the ray to the forms (a) and (b). 



4. The phloem tissues, which are best studied in sections 

 which have been treated for some hours with chlor-zinc-iodine, 

 consist of 



a. Sieve-tubes, elongated structures with cellulose walls, 

 those which are radial being marked by numerous circular 

 sieve-plates, here seen in surface view : these sometimes stain 

 a sherry brown with chlor-zinc-iodine. The ends of the tubes 

 are difficult to observe : their protoplasmic contents are trans- 

 parent and sparing. 



b. Phloem-parenchyma, cells arranged in longitudinal rows, 

 with cellulose walls, and copious protoplasm. 



c. Occasional elements (prosenchymatous or parenchymatous) 

 with brown cell-contents, in which crystals are embedded: 

 these are found towards the periphery of the phloem. 



Medullary rays will be seen with a similar arrangement to 

 that seen in the xylem. Their cells, which resemble those of 

 the phloem-parenchyma in character, are all alike. 



5. Externally to the phloem is the cortical parenchyma, 

 which requires no further notice here. Outside this is cork, 

 and at certain points a little sclerenchyma. At the periphery 

 of the section is the epidermis. 



V. Cut tangential sections of a three- or four-years-old branch, 

 and bear in mind that as a rule the central part of the sections 

 is the most accurately tangential, i.e. that the plane of section 

 is there most accurately perpendicular to the radius of the stem. 

 (See p 6.) 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 



