134 A PINE. 



of the bordered pits, characteristic of the wood of the group 

 of plants to which the pines belong. 1 



4. Outside the xylem, a thin layer of cambium tissue, seen 

 only in sections cut with extreme care from stems collected 

 during the growing season. 



5. The phloem, the whitish tissues outside the xylem and 

 cambium, composed of: 



a. Angular, whitish cells making up the greater part, 

 the sieve-cells. 2 



Compare the shape of active sieve-cells next the 

 cambium and those near the outside of the phloem, 

 which have become functionless with age. 



b. Near the periphery of the sieve-tissue an interrupted 

 row of cells with brown or yellow contents in which 

 are strongly refringent crystals. Near the cambium 

 a similar row of cells, larger and rounder than the sieve- 

 cells and with colorless or slightly yellowish homo- 

 geneous contents, in which a small crystal or two may 

 sometimes be seen. These two broken rows of cells 

 are the phloem parenchyma. 3 



6. The cortical parenchyma, lying just outside the phloem. 

 Observe. 



a. The shape, size, and arrangement of the cells. Com- 

 pare with the pith parenchyma in these respects. 



b. The contents of cells, including the distribution of 

 chlorophyll. 



1 For a clear idea regarding the structure of the bordered pits, as 

 well as for a knowledge of the structure and relations of the other parts 

 of the wood of the pine, see Strasburger's "Botanisches Practicum," 

 Coulter and Chamberlain's "Gymnosperms," and other text-books. 



2 So called because the radial walls of these cells are perforated by 

 clusters of very fine pits, the sieve-plates; they occupy the same relative 

 position as the bordered pits of the tracheids. 



3 These two tissues of the phloem can be well demonstrated by means 

 of differential stains. 



