CELLS AND TISSUES 33 



mosaic, and the relationship between sieve tube and companion 

 cell can be readily determined. On the other hand, there may be 

 much irregularity; and, where there is little difference in the size 

 and shape of the two elements, it is difficult to distinguish one from 

 the other except in longisections where the sieve plates can be 

 observed. 



The function of the companion cell is not definitely known; but 

 the fact that the sieve tube and companion cell are intimately con- 

 nected by plasmodesma, and that the sieve tube lacks an organized 

 nucleus while the companion cell has a conspicuous one, suggests 

 that the companion cell is concerned in some way with the con- 

 ductive function of the sieve tube. 



Phloem Parenchyma. — The parenchymatous cells of the 

 phloem do not differ histologically from those described under the 

 section devoted to parenchyma, but they may have an important 

 conductive function. This is especially true in protophloem in 

 which sieve tubes are frequently absent and the parenchymatous 

 cells constitute the chief conducting elements. These cells are 

 short-lived and are usually crushed and resorbed as the axis en- 

 larges. The metaphloem also contains some phloem parenchyma, 

 but sieve tubes and companion cells are differentiated. 



In the secondary phloem, the proportionate number of parenchy- 

 matous cells, sieve tubes, and companion cells is variable, the 

 former being quite abundant in many Compositae and almost com- 

 pletely lacking in some species of Ranunculaceae. Where phloem 

 parenchyma is derived from the cambium, the resultant cells are 

 frequently elongated and tapered, and have been termed cambijorm 

 parenchyma. In some instances, transverse divisions of these 

 elongated elements result in the formation of a linear series of 

 relatively short isodiametric cells. 



Ray parenchyma of the secondary phloem is also cut off by the 

 cambium and the phloem rays often lie on the same radii as the 

 secondary xylem rays, appearing as continuations of the latter out- 

 side the cambial ring. At its outer limit, the ray may be funnel- 

 shaped where it widens, owing to continued growth and division 

 of the parenchymatous cells comprising it. The ray cells are 

 commonly elongated in the radial direction, and may remain active 

 over long periods, sometimes through the life of the plant, provid- 

 ing a means of lateral conduction and also a place for storage of 

 food. 



