n8 THE ANATOMY OF WOODY PLANTS 



illustration appears the xylem, consisting of vessels, libriform 

 fibers, parenchyma, and wood rays. Above the tissue so organized 

 lies the cambium, or zone of growth, which alternately adds ele- 

 ments to the structure of the xylem and that of the phloem. The 

 latter tissue appears above the cambium in the figure and outside 

 it in the trunk of the tree. The preparation which served as the 

 basis of the illustration was made from material secured during the 

 period of winter rest. Next to the cambium lie fibrous and similar 

 elements, to the abundant development of which in its inner bark 

 the basswood (literally bastwood or tree useful for binding) owes 

 its name. The fibers of the phloem are often called hard bast. 

 Examination of the figure will show that the fibrous elements are 

 separated from the cambium by a row of cells either quite empty 

 or containing large crystals of calcium oxalate. These are the so- 

 called crystallogenous cells and are very commonly present inter- 

 nal to the zones of fibers or hard bast in the phloem of the form 

 under discussion. External to the hard bast lies a zone distin- 

 guished by the presence of cells either richly protoplasmic as to their 

 contents or, in case the protoplasm^ is more scanty, characterized by 

 relatively thin and unlignified walls. This region is the so-called 

 soft bast and shows considerable complexity of organization. 

 First taking the cells with abundant protoplasm, we see clearly that 

 these can be divided into two categories namely, those which 

 contain grains of starch (represented in black as if treated with 

 strong iodine solution) and those in which amylaceous substances 

 are absent. The latter are further distinguished by their generally 

 small size and somewhat triangular shape, due to the fact that they 

 are usually accommodated in the angles of large thin-walled ele- 

 ments with a delicate protoplasmic lining. The triangular cells 

 are known as "companion cells" and are a constant feature of 

 structure of phloem of the angiosperms as contrasted with the 

 gymnosperms. The larger elements of the phloem with thin walls 

 and scanty parietal protoplasm are the sieve tubes. In some in- 

 stances the sieve plate can be seen in the transverse section repro- 

 duced in the figure, and it is clear that its position is characteristically 

 radial. Farther outward the densely filled parenchymatous cells 

 of the soft bast give place again to the dead crystallogenous elements, 



