484 TRANSLOCATION OF SOLUTES 



elements mature the nuclei enlarge and disintegrate. The vacuolar sap of 

 sieve tube elements contains conspicuous amounts of proteinaceous colloidal 

 material. In mature sieve tubes the cytoplasm seems to be in a highly per- 

 meable condition to both water and solutes. 



Sieve tubes usually develop by the longitudinal division of sieve tube 

 mother cells which have arisen as a result of the division of a cambium cell. 

 The division of such sieve tube mother cells results in a sieve tube element 

 and a companion cell (Fig. 106). Sometimes the sieve tube mother cell may 

 divide longitudinally more than once producing one sieve tube element and 

 two or more companion cells. The companion cells are much smaller in 

 cross section than the sieve tube elements. They may have the same length 

 as the sieve tube element or they may be half or even less its length. 



The protoplasm of each companion cell contains a prominent nucleus, and 

 numerous small vacuoles, but no starch grains. The walls separating the com- 

 panion cell or cells from the sieve tube element are characterized by the 

 presence of numerous small pits but the walls between the companion cells 

 and the parenchyma cells contain few or no pits. The presence of these pits 

 suggests a close relationship between the companion cells and the sieve tube 

 elements. In some species some sieve tube elements do not have companion 

 cells adjacent to them while in other species every sieve tube element is ac- 

 companied by one or more companion cells. Gymnosperms and pteridophytes 

 do not have companion cells. 



Phloem parenchyma is composed of living cells which are somewhat 

 elongate parallel to the long axis of the stem. This tissue does not occur in 

 monocots and is also absent from the phloem of some dicots. The proportion 

 of phloem parenchyma cells in the phloem varies widely according to species. 

 In most herbaceous dicots they constitute a smaller proportion of the phloem 

 than do the sieve tubes and companion cells. The phloem of seedlings, how- 

 ever, may consist largely of phloem parenchyma. Woody species also differ 

 greatly in the proportion of phloem parenchyma cells present. The arrange- 

 ment of phloem parenchyma cells likewise exhibits a great variation. They 

 may occur in definite clusters, in tangential bands, or in radial rows that are 

 closely associated with the sieve tubes. Many of the phloem parenchyma cells 

 store starch and in certain species they often contain crystals. 



Phloem fibers, found in the phloem of some plants, are elongated cells 

 with thick, usually lignified walls. They are more common in woody than in 

 herbaceous species and, like the companion cells, they do not occur in the 

 gymnosperms or in the pteridophytes. The phloem fiber cells have long, 

 tapering ends which overlap forming strong fibrous strands. They frequently 

 occur in groups or as cylindrical sheaths surrounding the inner phloem tissues. 



