398 Farmer and Hill. — Arrangement and Structure 
in this, that we are thereby emancipated from the highly 
artificial considerations such as begin by regarding the pith 
as an integral part of the unit (the monostele), and then pro- 
ceed to disregard or deny its further existence as soon as 
the peripheral mantle of phloem, pericyele, or endodermis 
shall have extended ground the inner face of the xylem. 
Such a point of view seems to us to originate in and depend 
upon a one-sided and abstract contemplation of isolated 
transverse sections, and to omit all reference to the mutual 
relations of the tissues to each other, as continuous and con- 
crete realities in the body of the plant. By concentrating 
attention on the vascular strand as the thing of real import- 
ance, whether it be present in the form of a solid rod (proto- 
stele) or as a hollow cylinder (siphonostele), one is equally 
concerned with a definite unit, and the various transitions 
to polystely, &c., become clearly referable to a single plan, 
and their several relations become at once obvious. It is the 
inclusion of the pith as an integral part (which has to be 
subsequently discarded) together with a strained and artificial 
criterion as to the boundary of the stele, that seem to us the 
fatally weak points in the whole stelar theory, and which 
have resulted in unnatural interpretations being given in not 
a few instances of structures that do not conform to the more 
ordinary types of stem structure. 
It may, perhaps, be objected that inasmuch as all grada- 
tions may be traced between a xylem parenchyma that is 
intermingled with tracheids, and a true pith such as forms 
the core of a ( medullated monostele,’ this in itself constitutes 
a sufficient reason for not dissociating it from the vascular 
tissues in defining our conception of the stele. Similar objec- 
tions could probably be urged against most morphological 
distinctions, and certainly against all which are affected by 
tissue differentiation. The whole matter is really a question 
of consistency and convenience. If the inclusion of the pith 
in the stele is, however, no longer conducive to a clear 
comprehension of the latter in its comparative relationships, 
and still more if it can be shown rather to obscure them, 
