31 THE STRUCTURE OF ECONOMIC PLANTS 



enclosing a slime string of proteinaceous material and being itself 

 surrounded by a callus sheath lying between it and the margin of the 

 perforation. Crafts in a series of papers (li, 13, 14) reports that 

 the protoplasmic strands are probably solid, that they do not con- 

 tain pores through which slime strings extend; and suggests that 

 the "only continuous permeable phase throughout the plant is the 

 cell wall," which "is highly hydrated" in the phloem. He also 

 states that the permeability of the sieve tube increases as the ele- 

 ment matures ; and that "it seems possible that movement may take 

 place partly through the sieve-tube lumina and partly through the 

 phloem walls." 



A development which usually occurs late in the growing season, 

 or late in the ontogeny of the individual sieve tube, is the formation 

 of callus on the sieve plate. This at first surrounds the perforation, 

 later may completely cover it; and, in some cases, continues to be 

 deposited until a large mass of callus is formed. Later, it may be 

 dissolved off with a consequent resumption of the activity of the 

 sieve tube; but, in many instances, it forms a permanent layer 

 which terminates the conductive function of the element. 



Companion Cells. — The companion cell is so named because of 

 its intimate structural and probable functional relationship to the 

 sieve tube. In the ontogeny of the sieve tube and companion cell, 

 the two elements are derived from a common mother cell of the 

 procambial strand in primary phloem, or from a phloem mother cell 

 derived from the cambium in secondary phloem. A longitudinal 

 division of the phloem mother cell divides it into two daughter 

 cells of unequal size, the larger becoming the sieve tube and the 

 smaller the companion cell. The latter contains a very dense pro- 

 toplast with a distinct nucleus; and is thin-walled, elongated, and 

 triangular, square, or somewhat rounded in transection. The 

 walls adjacent to the sieve tube have numerous simple pits, and 

 there are a smaller number of them in the walls that are in contact 

 with parenchymatous cells. In many herbaceous angiosperms, the 

 length of the companion cell is equal to that of its sieve tube; but 

 the original companion cell may be divided transversely into several 

 shorter cells, so that each sieve tube lies adjacent to a vertical row 

 of two, three, or more short companion cells. 



There is variation with respect to the arrangement of the sieve 

 tubes and companion cells. In some instances, the pattern is so 

 regular that the phloem appears in transection as an almost perfect 



