112 CHEADLE 



ture. Secondly, he clearly demonstrated, and has continuously emphasized, 

 the interrelationship of structure and physiology of the sieve elements. With 

 this background of phloem study, his eloquent, imaginative, consistent de- 

 fense of the mass-flow hypothesis as an explanation of translocation in the 

 phloem has forced those who oppose it to broaden their own horizons in 

 coping with his arguments. Crafts' curiosity about the essential nature of 

 food-conducting elements led him to a relatively broad survey of brown 

 algae, bryophytes, gymnosperms, and angiosperms. From this survey came 

 some of the generalities about sieve elements that were given above in the 

 description of phloem. He particularly stressed the notion that the proto- 

 plasmic peculiarities of sieve elements make them unique among living cells. 

 From this conclusion came, almost inevitably, the concept that cells with all 

 or most of these characteristics in the bryophytes and the brown algae must 

 be sieve elements, even though xylem is absent. This occurrence of at least 

 sieve-element-like structures has important implications with regard to the 

 universal occurrence of some common structural peculiarities in food-con- 

 ducting elements and hence the relation of such structures to function; the 

 problem of relationship of vascular plants to brown algae and to the bryo- 

 phytes; and the presumed antiquity of phloem in contrast to xylem. All 

 these implications are of deep interest to those concerned with the biology 

 of phloem or with plant phylogeny. 



The greatest impetus to phloem study in America, however, has come 

 from Katherine Esau's research papers and reviews. Keen observer, meticu- 

 lous writer, excellent illustrator, linguist, her accomplishments in phloem 

 research and related studies have set the highest standards. An interest in 

 studies of the anatomic effects of sugar-beet virus and its movement in the 

 plant led naturally to broader and broader investigations of phloem. Once 

 again, studies intended originally to be chiefly practical in outlook turned 

 almost immediately into fundamental investigations upon which all practical 

 applications appear to depend. It is almost axiomatic that we quickly exhaust 

 our basic information when it is needed in the detailed solution of more 

 obviously practical problems. 



Esau's knowledge of the literature has led to clarification of terminology 

 as well as a sound basis for various approaches to study of the phloem. The 

 terminology in Cheadle and Whitford's papers on the evolution of sieve 

 tubes in the monocotyledons in 1941 and subsequent years, for example, is 

 primarily an outcome of conversations with her. Utilizing her understanding 

 of developmental anatomy, she made detailed investigations of the phloem 

 of tobacco, grape (including seasonal changes), and other plants. She de- 

 scribed the unique release of nucleoli from nuclei during developmental 

 stages of sieve elements in a number of plants. Esau took a major part in a 

 study on plasmolyzability of sieve elements that settled a controversy of 

 long standing over this aspect of sieve elements. In later years, with her 



