No. 464.] STUDY OF THE SALICACEE. 527 
impossible to compare such plants in any direct way with the 
polystelic angiosperms in which the mechanical tissue generally 
encloses the vascular structure in a more or less definite sheath 
which exhibits a more or less clearly defined relation to mechan- 
ical stress. While, therefore, for the purposes of our present 
argument, it is impossible to introduce exact comparisons with 
any of the polystelic forms of stems, we must nevertheless con- 
clude that the secondary wood, in whatever form it may be dis- 
posed, must be regarded as differentiated to meet mechanical 
ends as shown by Williamson (71) in his various elaborations of 
the Calamitean structure and as more recently stated by White 
(105); but a knowledge of the relations existing between the 
protoxylem and the secondary xylem requires a more critical 
examination in detail, in order to determine the course of devel- 
opment which has given to the gymnosperms and the dicotyle- 
donous angiosperms, the peculiar structural aspects distinguishing 
them respectively. This will constitute the subject of a special 
paper ata future date, and in the meantime it will serve our 
present purpose to state briefly the fundamental conceptions 
which form the essential features of our hypothesis. In order 
to make our point of view more clear, it will first be necessary 
to state concisely what has been shown to hold true of the 
gymnosperms with respect to the evolution of the secondary 
vascular structure and its economic aspects in relation to the 
requirements of support, as well as its physiological ré/e in the 
conveyance of nutrient fluids. 
The origin of the gymnosperms in the Cycadofilices, or at 
least a portion of them, is now generally accepted as an estab- 
lished fact. The essential starting point for the evolution of the 
vascular cylinder is therefore to be found in the scalariform and 
spiral vascular elements of the Cycadofilices or their filicinean 
ancestors which constitute the entire vascular bundle of those 
plants, and which, in the gymnosperms, constitute the protoxy- 
lem structure only. The relatively greater extent to which the 
mechanical elements are developed in some of the Cycadofilices 
and in the Cycadacez as a whole, is in direct response to the 
somewhat more arborescent tendency of these groups as partic- 
ularly expressed in certain genera; while the fact that they do 
