358 
I. W. BAILEY 
Magnoliaceae, text fig. i), but also by the fact that the tracheary cells in 
the dicotyledons tend to shorten as the vessels become more and more 
highly specialized (text fig. 2).^ 
In all of the arborescent dicotyledons and gymnosperms, with the 
probable exception of the Cordaitales, Bennettitales, and Cycadales, the 
first formed tracheary cells of the secondary xylem are relatively small 
and are considerably shorter than the adjoining elements of the primary 
xylem (text fig. i). This is in marked contrast to the conditions which 
appear to have prevailed in the stems of many of the lower vascular plants. 
In forms having relatively wide zones of primary wood, the innermost 
secondary tracheids resembled in size the outermost primary tracheids. 
It seems probable that in the evolution of the higher gymnosperms and 
dicotyledons, with reduction in the amount of primary xylem and with 
other changes in the innermost portion of the stele, there has been a con- 
comitant shortening of the first formed elements of the secondary xylem. 
The size of the cells in the secondary xylem is determined by (i) the 
rsize of the cambial initials, and by (2) changes that take place in their 
derivative cells during differentiation into tracheary elements. It is 
conceivable, therefore, that the variations in the size of the tracheary 
elements may be closely correlated with similar fluctuations in the size 
of the meristematic cells. It is also conceivable, however, that the cells of 
the lateral meristem are of relatively uniform size, as hypothesized by 
Strasburger (1893), Winkler (191 6), and others, and that the differences in 
the size of tracheary cells are due entirely to changes, e.g., expansion, 
division, etc., which occur during differentiation of the xylem. The present 
paper is devoted to a comparative study of the size variations of cambial 
initials and tracheary cells. 
Material and Methods 
There are two methods of determining the sizes of the cells in a given 
tissue: by measurements taken (i) from sections and (2) from macerations. 
Each method has certain inherent advantages and disadvantages. In 
macerations it is possible to isolate individual cells and measure their 
various dimensions, but it is necessary to allow for differences in breakage, 
^ The secondary xylem of the Calamariales, Sphenophyllales, Lepidophytineae, 
Cycadofilices, and Gymnospermae, exclusive of the Gnetales, is comparatively homogeneous, 
and its tracheary cells are of a single generalized type, so-called tracheids. In the Gnetales 
and Dicotyledoneae specialization or "division of labor" appears to have occurred among 
these cells. Certain vertical series of tracheids have become modified and function prin- 
cipally in conducting liquids, whereas others have assumed a mechanical role. As the 
vessels of the dicotyledons become more and more highly differentiated, their segments 
change their shape and structure and lose their resemblance to tracheids. At the same 
time, the surrounding tracheary elements tend to take on a more fiber-like structure, their 
pits becoming vestigial by the gradual disappearance of the bordering areas in the secondary 
walls. 
