RESEARCH ON XYLEM AND PHLOEM 97 



a manner apparently common to all) what no longer seems important in their 

 survival. 



The occurrence of xylem and phloem has been so evidently associated with 

 the land habitat that when vascular plants are mentioned, it is understood 

 that typical land plants are meant. The importance of vascular tissues has 

 been emphasized in recent decades by use of a group name for all of them — 

 the Tracheophyta (or Tracheata). This huge group of plants consists of a 

 wide variety of forms, but all of them have vascular tissues — absence of them 

 represents evolutionary loss. Among these plants are the present-day club 

 mosses, horsetails, quillworts, ferns, gymnosperms, and angiosperms. Added 

 to these groups, each of which has additional features that separate them one 

 from the other, are those curious genera Psilotum and Tmesipteris, plants 

 which give us some insight into what must have been the earliest land plants — 

 though they themselves are considerably removed from these ancestors. Still 

 more fascinating members of the Tracheophyta are the extinct groups: the 

 Psilophytales and their primitive cohorts, giant horsetails and club mosses, 

 primitive ferns, all seed ferns, many conifers, and the like, which at one time 

 or another flourished and were lost, never to be known again except as fossils. 

 And we have not yet more than a well-founded suspicion of what went on 

 before these vascular plants, even the primitive Psilophytales with their lack 

 of roots and leaves, arrived on their scenes. The antiquity of these forms can 

 be partially recognized when one recalls one of Professor E. C. Jeffrey's apt 

 expressions, "The pines are older than the birds that nest in their branches." 

 And birds arose tens of millions of years before Man ever heard their songs. 



Hundreds or thousands of fossil species, many hundreds of species in the 

 lower vascular plants, thousands of ferns, hundreds of gymnosperms, scores 

 of thousands of species of flowering plants — these are the materials that con- 

 front one who is interested in vascular tissues. These are the materials with 

 endless variation in ontogeny and in construction of their mature vascular 

 tissues — details that are often available for study only after painstaking 

 preparation for observation. 



In spite of the difficulties inherent in the study of vascular tissues, much is 

 known of them ; so much in fact that I hesitate in choosing those accomplish- 

 ments that might exemplify the progress of research on vascular tissues, par- 

 ticularly in America, in the last fifty years. I have nevertheless arbitrarily 

 chosen some lines of research I am most familiar with and another which 

 recently has become lively. 



Before proceeding to these, however, it may be useful to write briefly of the 

 variation of patterns of conducting tissues that can be observed throughout 

 the vascular plants. These patterns actually represent, of course, the occur- 

 rence of vascular tissues, the subject of this paper. Their interest here, how- 

 ever, lies more in the way they serve as a means of illustrating a phase in the 

 development of knowledge about structure and the motivation which led to 



