242 PRACTICAL BOTANY 



the uniform cambium of the ray to the forms (a) 

 and (6). 



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. 



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 transparent and sparing. 



&. Phloem-parenchyma, cells arranged in longi- 

 tudinal rows, with cellulose walls, and copious proto- 

 plasm. 



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- 

 chyma, 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 



