RESEARCH ON XYLEM AND PHLOEM II5 



attract the attention of morphologists, for in this type of growth we have 

 one of the most essential differences between typical animals and plants. If 

 we think about this growth for a moment, one of the striking facts that 

 should occur to us is that in moving downward, so to speak, from the apex of 

 an actively growing shoot of a tree, for example, we soon pass through all 

 the stages that represent longitudinal growth of the stem; thereafter what 

 we observe is chiefly related to increase in girth. What may not often occur 

 to us in such observations, perhaps, is that we have the possibility of study- 

 ing the interplay between whatever influences may reside in young undif- 

 ferentiated cells on the one hand and more mature assemblages of cells on 

 the other. There is, in addition, the effect of cells in various stages of dif- 

 ferentiation to be considered. It should be obvious, accordingly, that a clear 

 understanding of the changes in the various regions of a young tip in a normal 

 plant must precede any understanding of what may occur when normal 

 growth of the tip is disturbed. Most of the research up to recent times has 

 been devoted to the development of an understanding of the normal sequence 

 of events in primary growth of shoots. 



Through the efforts especially of A. S. Foster and some of his students and 

 colleagues, we have learned from research in this country much about the 

 make-up of the shoot apex. We know from this work, for example, that the 

 meristematic cellular constituents of the shoot apex are not haphazardly ar- 

 ranged, but that they have, within certain limits, a recognizable organization. 

 Furthermore, the patterns of organization are even arranged by some writers 

 in several loosely defined categories, although these are not mutually exclu- 

 sive. It is helpful to be aware of the cellular organization in the shoot apex, 

 because descendants from cells in these apices indirectly produce the leaves 

 and eventually become differentiated as the primary body of the stem. In 

 this connection, the precise relationship between the initial vascularization — 

 formation of procambium and later differentiation of the xylem and phloem 

 — of the stem and that of the leaves developing from it may be of the greatest 

 value to experimental morphologists. Important as information concerning 

 the shoot apex proper is, however, the descriptive research concerning it ordi- 

 narily has not considered derivative groups of cells far enough down the stem 

 to provide adequate information on the ontogeny of primary vascular tissues. 



In this country, the earliest important emphasis on primary vasculariza- 

 tion — development of the primary vascular tissues — again appeared in Esau's 

 papers. In attempting to clarify the "host-virus relations in the curly top dis- 

 ease" in tobacco, she found it necessary to undertake a critical analysis of the 

 ontogenetic history of primary vascular elements. It became essential, too, 

 for her to distinguish between primary and secondary elements, as well as 

 between procambial cells (direct precursors of primary vascular elements) 

 and other meristematic cells. Esau later answered her own plea for further 

 critical research of this type by detailed studies of a number of species. From 



