Exercise XI 



SIEVE-TUBE ELEMENTS 



I. Introduction. — The phloem of vascular plants, like the xylem, 

 is a "complex tissue" which may consist of four or five dif- 

 ferent types of cells. Nevertheless, the phloem is morphologi- 

 cally well defined by the consistent presence of a highly special- 

 ized kind of cell know^n as a sieve-tube element or sieve-cell. This 

 definitive cell type lacks a nucleus at maturity and in addition 

 possesses other structural and physiological properties which are 

 apparently unique. In many angiosperms, the sieve-tube ele- 

 ments are clearly arranged in definite vertical series, to each of 

 which the collective term "sieve-tube" may be applied. The 

 term sieve-tube has also been given to the individual enucleate 

 cells typical of the phloem in gymnosperms. Such cells, how- 

 ever, are not arranged in linear series. Abbe and Crafts (1939) 

 refer to these structures in conifers as "sieve elements." As 

 Esau (1939) has pointed out in a thorough review of the litera- 

 ture on phloem structure, the word "sieve-tube in its original 

 meaning had reference to a series of superposed cells with trans- 

 verse or somewhat inclined end walls bearing sieve-plates." 

 Under such circumstances, the term "sieve-tube" will be reserved 

 in this book for definable vertical cell-series and the term "sieve- 

 tube element" used for the individual members of such a series. 

 Similar cells of the phloem of gymnosperms, not arranged in 

 vertical superposed series, will be designated as ' ' sieve-cells. 



In angiosperms, the sieve-tube elements are usually accom- 

 panied on one of their lateral walls by small prismatic or tubular 

 cells. These cells, which are intimately connected with the sieve- 

 tubes, are termed companion cells and differ from the sieve-tube 

 elements in possessing nuclei and in lacking definite sieve plates. 

 Companion cells are to be regarded as sister cells of the sieve- 

 tube elements since both types of cells originate by the division 

 of a common mother cell (cf. Esau, 1939, pp. 409-410). Sieve- 



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