RESEARCH ON XYLEM AND PHLOEM III 



degenerate nuclei in Isoetes) ; and it is at the stage when these typical features 

 are evident that the elements presumably become functional in rapid con- 

 duction of food materials. It appears, then, that the functioning of these 

 elements is associated with their cytological condition. If true, it is obvious 

 why the cytology of sieve elements should be of interest to physiologists and 

 why cytological details should be accurately reported. 



There are still other details of sieve elements that merit brief mention. 

 Slime and additional details of the wall structure are examples. Slime is 

 noted here chiefly because it often occurs abundantly in the sieve elements 

 of those plants used for class study (e.g., cucurbits). It appears in various 

 forms, and these may represent seasonal changes or stages of development of 

 the sieve elements. The additional feature of the wall in the sieve elements 

 of many species of plants concerns variability in thickness. In fresh sections, 

 the thicker walls, especially, often have a pearly luster, hence the name 

 nacre applied to such walls. There is little specifically known of this feature 

 of sieve elements, and hence nothing more will be said of it at this juncture; 

 it will be referred to briefly again later. 



The early important information on phloem from American laboratories 

 came from MacDaniels, whose paper in 1918 on phloem of woody plants 

 corrected Hemenway's observations and unsubstantiated notions on the 

 evolution of sieve tubes in relation to the phylogeny of the dicotyledons. It 

 should be remembered that in 1918 the important work on the vascular 

 cambium and the length of its xylem derivatives had not yet been published. 

 Hence MacDaniels' accurate observations could not be adequately inter- 

 preted against a background of reasonable generalizations about the cambial 

 derivatives. Nevertheless, it is evident from the early work that in the evolu- 

 tion of sieve-tube members they not only became shorter, but also their end 

 walls became more nearly transversely placed (fig. 16, 18). 



As was mentioned previously, an American — Katherine Esau — is the best- 

 informed investigator of phloem structure. Her entrance into this field was 

 primarily motivated by an interest in the translocation and effect of virus 

 particles. And Alden S. Crafts, the leading physiologist in the field of trans- 

 location in this country, pursued research in phloem anatomy in the effort to 

 learn more about the mechanism of translocation. In fact, aside from a very 

 few papers, the important American literature on phloem is primarily a re- 

 flection of curiosity about vertical conduction in the phloem. This is under- 

 standable because of the importance of translocation in terms of nutritional 

 relationships in the plant as a whole and of the movement of other sub- 

 stances, such as herbicides and viruses, with the translocation stream. The 

 latter two materials are dealt with by other authors of these Jubilee papers. 



Crafts' major contributions to phloem anatomy, introduced at about the 

 time that K. Esau commenced her studies, were twofold. In the first place, 

 he brought into modern focus a good deal of European data on phloem struc- 



