Il6 CHEADLE 



these studies came an understanding of the relation of appearance and size 

 of vascular tracts in the stem to phyllotaxy (arrangement of leaves). They 

 demonstrated, also, the difficult and painstaking nature of this type of 

 research. Her students and many of Foster's have since completed other 

 investigations of this type. Wetmore and his students also early followed a 

 comparable type of research in a greater variety of vascular plants before 

 turning chiefly to the extremely valuable experimental work that has char- 

 acterized publications from his laboratory in recent years. 



It seems abundantly clear from the comments just made that a thorough- 

 going knowledge of origin of procambium in stems and leaves, both in rela- 

 tion to time and position, together with a comparable understanding of the 

 differentiation of primary xylem and phloem, is difficult to achieve. It is 

 just as clear, moreover, that any departure from a normal sequence of events 

 cannot even be recognized as such without this knowledge and understanding. 



It is a tribute to the professional courage of experimental morphologists 

 that in spite of these obstacles, they have launched into programs of exciting 

 research on shoot tips (see other contributions in this Jubilee series). By the 

 use, where appropriate, of tissue-culture techniques developed in the latter 

 decades of this half century — these in turn were based upon nutrient culture 

 techniques involving whole plants — and of surgical procedures, the way 

 has been opened not only for studies of structure in relation to growth, but 

 also for a thoroughgoing program of biochemical research in connection with 

 them. 



We are thus on the threshold of an era in which much can be learned 

 about the causal relationships of form in plants, about various mutual in- 

 fluences of differentiated and undifferentiated groups of cells. The literature 

 to date has shown that some of the details of vascularization of the shoot, 

 especially in relation to the leaves, can be used as important testing devices 

 for determining the causal influences we have briefly mentioned. Those of us 

 who by inclination or opportunity find our own research interests in other 

 aspects of morphology hope to gain much by rapid progress in these experi- 

 mental aspects of structure and form in plants. My plea is that we do not 

 slow down essential progress by excessive speculation based upon incomplete 

 knowledge of initial vascularization in normal plants and by crowding the 

 literature with unsatisfactory, if not actually misleading, casually descriptive 

 papers purporting to serve as the bases for such speculations. It seems obvious 

 that carefully prepared descriptions of adequately investigated typical plants 

 will provide the only sound basis upon which to interpret experimentally 

 induced variations. At the same time, such descriptions will add to our 

 inadequate knowledge of structure in normal plants. 



Conclusion. The remarks just concluded on certain arbitrarily chosen 

 studies of vascular tissue chiefly referred to the individuals and the intel- 

 lectual motivations concerned with these studies during the past fifty years. 



