PINUS. STEM. 147 



3. The Cambium-layer consisting of elongated thin- 

 walled cells, the ends of which are difficult to observe 

 (cf. tangential sections). They have copious protoplasm, 

 and an elongated nucleus. 



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 (ft). 



In the sections through the cambium of a stem cut in 

 summer, the development of the bordered pits on the walls of 

 the tracheides may be studied. The sections should be treated 

 with Schulze's solution for a long time. 



4. The Phloem tissues, which are best studied in 

 sections, which have been treated for some hours with 

 Schulze's solution, 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 Schulze's solution. 

 The ends of the tubes are difficult to observe (cf. 

 tangential sections). Their protoplasmic contents are 

 transparent and sparing. 



I. Bast-parenchyma, cells arranged in longitudinal 

 rows, with cellulose walls, and copious protoplasm. 



c. Occasional elements (prosenchymatous or paren- 

 chymatous) 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 arrange- 

 ment 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 paren- 



L 2 



