114 CHEADLE 



merits — our general ideas will almost certainly be changed by study of tropi- 

 cal and semitropical dicotyledons and of perennial monocotyledons in all 

 climates ; the occurrence and number of companion cells and whether they are 

 lacking in any significant number of species — time-consuming and difficult 

 research; the frequency and precise positional relationships of various other 

 parenchyma cells; and the influence of fiber distribution on longevity of 

 sieve elements. It may be, too, that the evolutionary development of the 

 various types of sieve elements may supply critical information on the 

 physiology of food-conducting cells. 



In connection with the size of the lumen of the sieve element, through 

 which food substances presumably pass, it would be wise for students of 

 translocation to consider the extraordinary thickness of walls of sieve ele- 

 ments in many species. Even after complete dehydration, many of these 

 walls are so thick that the lumen is almost completely obstructed. This 

 aspect of structure in relation to translocation is almost universally ignored, 

 chiefly because such thick walls are not commonly present in the usual experi- 

 mental plants. Here is a problem that needs critical attention. 



Fibers and sclereids need further study, particularly of their ontogenetic 

 variations and of their special relation to other elements in the phloem. The 

 ontogeny of fibers that occur in what has ordinarily been assumed to be the 

 pericycle needs to be studied in a wide variety of angiosperms, particularly, 

 for these fibers actually originate in primary phloem in many species and 

 perhaps in most. 



Truly the field of phloem anatomy is a wide-open one. We need more 

 investigators; only a few botanists, including Artschwager and, more re- 

 cently, Schneider, seem interested in solving the problems presented by this 

 extraordinary assemblage of cells. Those of us in this field invite our col- 

 leagues to join in the quest for knowledge — there are problems enough for 

 scores of researchers in this country in addition to those active in other parts 

 of the world. 



Initial vascularization. I should not close this article on vascular tis- 

 sues without some comment on concepts of the initial vascularization of the 

 shoot — an important aspect of development of primary tissues in general. A 

 critical evaluation of the research on this subject in the decade preceding 

 1954 has been given by Esau, and earlier work has been reviewed by Wetmore 

 and others. The following, accordingly, is a brief summary made from a dif- 

 ferent point of view and is not meant in any sense to be critical. The com- 

 ments will lay stress on the individuals involved and the importance of our 

 fund of basic morphological information to those who must use it in inter- 

 preting experiments designed to extend our understanding of causal rela- 

 tionships in morphology. 



It is not surprising that the more or less indefinite apical growth of the 

 vegetative shoots (and roots, for that matter) in vascular plants should 



