I08 CHE ABLE 



should acknowledge the beautiful work on cell walls by Bailey and Kerr 

 and others and the huge literature on wood identification and wood technol- 

 ogy by numerous workers in this country, to say nothing of those abroad. 

 But the progress in these and other important fields in the last fifty years 

 must be told by others. 



Phloem. The second general area of research I should like to review con- 

 cerns phloem. In spite of the fact that xylem and phloem are intimately 

 related in vascular plants, progress on investigations of the structure of 

 phloem has been slow. There are several reasons for this. In contrast to the 

 xylem, the phloem has soft conducting cells which are commonly crushed 

 as new secondary tissues are produced by the vascular cambium. As a mat- 

 ter of fact, the outer (older) phloem in most older stems is successively 

 sloughed off by the formation of cork by cork cambia. Again in contrast to 

 the conducting cells in xylem, the actively conducting cells of phloem are 

 living, or are at least in a labile form generally associated with the living 

 state, and thus many aspects of phloem cannot be studied in dried material 

 — the extensive usable wood collections over the world are not matched by 

 comparably useful bark collections. Likewise, herbarium materials cannot 

 often be effectively used in phloem studies, although they frequently can 

 in researches in xylem. These same contrasting features of phloem and xylem 

 are related to the paucity of good fossil material of phloem and the rela- 

 tively frequent appearance of fossil remains of beautifully preserved xylem. 

 To compound the difficulties, the ontogenetic changes that occur both with 

 regard to cell divisions and to cellular content are much more complicated in 

 phloem. The foregoing peculiarities of phloem, when added to the hetero- 

 geneity of the tissues as often represented by mixtures of hard- and soft- 

 celled constituents, make technical preparation itself a formidable obstacle 

 to progress. 



It is paradoxical, in one sense, that although our understanding of phloem 

 is not equal to that of xylem, there have been more recent and more widely 

 disseminated reviews of both structure and function of phloem than there 

 have been of xylem. The recent accounts by K. Esau and by A. S. Crafts in 

 the Botanical Review and by Esau and others in the American Journal of 

 Botany are especially worth reading. Another aspect of the literature on 

 phloem should be stressed at this point. Although the best-informed investi- 

 gator of phloem is undoubtedly an American scholar, there have been more 

 publications abroad on this tissue — and this is true recently as well as in the 

 past — than in the United States. The summary of important research ac- 

 complishments in the last fifty years in this country will consequently not be 

 as representative of the total new knowledge of phloem as was that for the 

 xylem. After a rather lengthy orientation, accordingly, a condensed review 

 will be given of certain aspects of phloem research. 



In relation to the subject matter arbitrarily chosen for discussion of xylem, 



