504 Bailey and Thompson.—Additional Notes upon the 
and containing a fluid. If this remains still closed after its development is 
completed, it is called a cell, cellula , but if a row of utricles arranged in 
a line become combined, during development, into a tube with an unin¬ 
terrupted cavity, through the absorption of their cross walls, a compound 
elementary organ is produced—the vessel (spiroid of Link).’ 
The importance of von Molds developmental studies was appreciated 
by many of his contemporaries, and his distinction between vessels, compound 
structures, and simple tracheary cells has been accepted, in but slightly 
modified form, by subsequent writers, including Caspary, Sanio, de Bary, 
Sachs, Strasburger, Van Tieghem, Scott, and Coulter. 
Thus, in its generally accepted modern meaning, a vessel, Gefass , 
trachea (Sanio), is a compound structure that arises from a series of cells by 
the loss of the pit membranes in the division walls between the members of 
the series, the latter being termed the members or segments of the vessel. 1 
The traumatically produced structures, described and figured by 
Jeffrey and Cole, are not compound structures, but simple cells, having pits 
with membranes and well-developed bordering areas (PI. XVI, Fig. 9). 
Therefore they are not vessel-like in structure. 
The next question to be considered is, are these tracheary cells seg¬ 
ments of vestigial vessels? If they are to be considered as such, there 
must be some criterion for distinguishing them from ordinary tracheary 
elements. In macerations of the secondary wood of plants which possess 
vessels, it is possible to separate the segments of the vessels from tracheides, 
fibre-tracheides, and libriform fibres by their size, form, or the structure of 
their pitted walls. Thus, in the majority of the Dicotyledons, the vessel- 
segments are not only noticeably larger in diameter than the neighbouring 
tracheary elements, but show unmistakable evidences of having been joined 
together to form a segmented tube or ‘ duct ’ (Text-figs. 6-9). Even in those 
exceptional cases where the segments of the vessels resemble the surround¬ 
ing tracheary cells in general size and shape, they can readily be distin¬ 
guished from them by the structural peculiarities of certain of their pits, 
which are perforated and without well-developed bordering areas. 
The cells which occur in the root of Drimys resemble the surrounding 
tracheides in general size and shape, and are quite unlike the typical vessel- 
members of Euptelea , Illicium , K ad sura , Schizandra , Michelia , Magnolia , 
Talauma , and Liriodendron (Text-figs. 1-9). Furthermore, these traumati¬ 
cally produced elements possess throughout pits with well-developed 
bordering areas and membranes, which serve to separate them quite sharply 
from the vessel-segments of the genera just enumerated. 
Jeffrey and Cole placed much emphasis upon the fact that elongated 
1 To refer to a single cell as a vessel, as has been done by Jeffrey and Cole throughout their 
paper, appears to be somewhat unfortunate, and likely to lead to unnecessary confusion. 
