igig] THOMPSON— COMPANION CELLS 455 



companion cells of angiosperms, as is shown by their small size, 

 their association with sieve tubes, their location in the corners 

 of the sieve tubes, their lack of starch, and their great elongation 

 in the vertical direction. Storage parenchyma of the ordinary 

 type (not rays) as found in angiosperms is absent. Sieve tubes 

 and companion cells are arranged with great regularity, the former 

 in very uniform rows and the latter in the angles between the 

 tubes. Usually there is an interrupted row of companion cells 

 for every row of sieve tubes, but occasionally the former are 

 lacking, as rriay be seen toward the right of fig. 3.^ 



The development of the bast at the cambial region is illustrated 

 in fig. 4. It will be seen that the companion cells are formed in 

 radial rows which are continuous through the cambium, and that 

 in these rows sieve tubes are not formed. There is a tangential 

 alternation of cambial rows which form sieve tubes with cambial 

 rows which form companion cells. Both are never formed in the 

 same row. As the bast matures and the sieve tubes expand, the 

 rows of companion cells become interrupted and the individual 

 cells pushed to the corners of the sieve tubes. 



This process is evidently quite different from that which occurs 

 in angiosperms, because in the latter sieve tube and companion 

 cell are invariably successive cells in the same row. There is no 

 separate cambial mother cell for the companion cells. Thus the 

 similar mature condition is brought about in quite dift'erent ways 

 in the two groups. 



A study of the wood adjacent to the cambium, also illustrated 

 in fig. 4, reveals an unexpected relationship between the companion 

 cells and the parenchyma of the wood. The same cambial rows 

 which form companion cells outwardly also form rows of wood 

 parench}Tna cells inwardly. The latter with their protoplasm 

 are easily distinguished in the figure from the empty, thick-walled 

 wood fibers which are formed in rows by the same cambial cells 

 which form sieve tubes. The rows of wood parenchyma formed in 

 this way are readily mistaken for uniseriate rays, but longitudinal 



» It may be remarked that the bast of Gnctum is extremely favorable material, 

 both for original study and for class use. The sieve areas are remarkably abundant 

 and of great size, and the individual pores are very large and clear. Moreover, the 

 whole tissue and the connected cambium are not easily crushed in sectioning. 



