154 BAILEY AND TUPPER. 



names and authorities. Of course, these specimens are available for 

 future study and comparison. 



In dealing with a' considerable number of specimens from various 

 parts of the world, it is extremely difficult to be certain that every 

 specimen has been assigned to the right species, and in some cases 

 to the proper genus. Such errors may be extremely serious in certain 

 types of investigations, but, as will be evident later, can have no 

 appreciable effect upon the general conclusions reached in this paper. 

 The plants are grouped into orders, families and subfamilies, accord- 

 ing to the Engler and Gilg (1912) system of classification. 



For the purposes of this investigation, simple and septate libriform 

 fibers, as well as fiber-tracheids, tracheids and vessel-segments, 

 are classified as tracheary elements. Fiber-tracheids and libriform 

 fibers grade into one another and appear to be interchangeable in many 

 plants. That is to say, in certain specimens or growth layers there 

 are clearly differentiated bordering areas about the pits, whereas in 

 others thay have almost if not entirely disappeared. Furthermore, 

 it frequently is difficult to determine whether vestigial borders are 

 present or not. 



Lengths of tracheary elements were obtained from macerations; 

 widths of vessel-segments, and notes in regard to the general structure 

 of the xylem, from microtome sections ten micra in thickness. All 

 measurements, from 10-100 depending upon the accuracy desired, 

 were made with a standai'dized micrometer eyepiece. Averages of 

 50-100 measurements were used as basic points in plotting the graphs. 

 The means given in Tables I and II are usually averages of 10 measure- 

 ments. The probable errors of these means fluctuate between two 

 and six percent. 



Size Variation of secondary tracheary Elements in Stems 

 OF Gymnosperms and Dicotyledons. 



In Tables I and II are recorded the lengths of the tracheary elements 

 of a considerable number of gymnosperms and woody dicotyledons. 

 Since the specimens, from which the measurements were made, were 

 secured from chfl'erent parts of stems, from plants of diO'erent ages, 

 sizes, growth habits and systematic affinities, and from dill'erent types 

 of environments in various phytogeographic regions of the earth, these 

 data should give a fairly reliable indication of the general size varia- 

 tions that occur in the tracheary elements of the secondary xylem of 

 the stems of conifers and arborescent and fruticose dicotyledons. 



