RESEARCH ON XYLEM AND PHLOEM IO9 



it was shown that the intellectual drive involved in the reviewed research on 

 wood lay in curiosity about the evolutionary development of various kinds of 

 cells and assemblages of them. This has not been the chief motivation in 

 the study of phloem in this country. The principal objective in published re- 

 search on phloem has been a more complete understanding of its functioning 

 as the chief food-conducting tissue. As a matter of fact, many of the detailed 

 papers on phloem anatomy have appeared as side issues, more or less, to the 

 main objective of understanding phloem physiology. It is worth stressing 

 that recognition of the relationship between structure and function has had a 

 sobering effect on physiological speculation. 



As pointed out earlier, we do not have a large fund of information on some 

 aspects of phloem, and this is especially true in terms of comparative studies. 

 And in spite of a considerable body of data on the physiology of conduction 

 of food in the phloem, there is still controversy over the mechanism of 

 translocation of these materials. Perhaps because of this, interest has cen- 

 tered on the cytophysiology and structure of the principal conducting ele- 

 ments themselves. As we make our short inventory of the cellular components 

 of the phloem, let us keep in mind the importance of these elements, so un- 

 usual that Katherine Esau has referred to them singly as "the nonconformist 

 plant cell." 



Disregarding for the moment the unusual food-conducting cells in cer- 

 tain primitive (nonvascular) plants, the vertically conducting elements in 

 the phloem consist of sieve cells and sieve tubes. Sieve cells (fig. 15) occur 

 in most of the vascular plants below the angiosperms, whereas sieve tubes, 

 composed of longitudinally oriented series of cells known as sieve-tube mem- 

 bers (fig. 16, 18), occur in the angiosperms. Parenchyma cells are also im- 

 portant constituents of the phloem, and they occur in varying degrees of 

 relationship to the sieve cells or sieve tubes. Certain parenchyma cells, for 

 example, arise from a phloem initial that immediately thereafter becomes a 

 young sieve-tube member and are known as companion cells (fig. 17, 18) 

 because of their close relation to these conducting cells. In addition to paren- 

 chyma, there are often fibers, or sclereids, thick-walled cells that tend to 

 make the phloem in some species exceedingly hard or tough. Disregarding 

 some unusual cells or cell complexes, the final constituent to note is the 

 phloem ray, previously mentioned when the radially disposed sheets of cells 

 in secondary vascular tissues were casually described. 



The sieve elements (this term is used when sieve cells, or sieve tubes, 

 or sieve-tube members are meant) have a combination of features that, taken 

 as a whole, make them unique among plant cells. The most common features 

 may be given in a brief description of the ontogeny of sieve elements. 

 Whether such elements arise in the primary body of the plant or in the sec- 

 ondary, they of course at first have a nucleus. In most plants, this nucleus 

 normally disintegrates during maturation of the sieve elements, although the 



