534 Boodle . — Comparative A natomy of the 
‘ stele * and ‘ monostely ’ in a morphological sense, applying as 
strict morphology as is possible in the case of tissues, and 
working on the hypothesis set forth in the present paper. 
As a consequence of this conclusion the description and 
classification of different types of stelar structure have at 
present no morphological basis, but only a physiological one, 
because they refer to specializations of tissue within the 
morphological unit with which we started. 
Brebner (’ 02 , p. 548) recognizes the physiological nature 
of the descriptive terms, which he applies to different types of 
stelar structure. In the writer’s opinion, also, terms defined 
as referring to definite types of tissue-arrangement within the 
stele are useful, and in some cases necessary, but a constant 
morphological distinction between the different kinds of tissue 
concerned is not upheld. As to the terms to be employed 
many already in use are sufficiently suitable. Thus ‘ soleno- 
stele ’ as defined by Gwynne- Vaughan (’01, p. 73) describes 
a special arrangement of tissues ; its derivation signifies ‘ tube- 
stele,’ and whether one regards the stele itself as being tubular, 
or the vascular part of the stele as being tubular, does not 
interfere much with the appropriateness of the term. 
Farmer and Hill (’ 02 , p. 398, &c.) decide to take the vascular 
strand as their unit for comparative considerations, both pith 
and the parenchyma forming the leaf-gaps of a solenostelic 
or dialystelic type being excluded. This is excellent as a 
physiological treatment of the tissues, but, in accordance with 
the views adopted in the present paper, the writer holds that 
it obscures the homologies of the tissues concerned. Leaf- 
gaps are held to have been originally formed by the replace- 
ment of vascular tissue by ordinary parenchyma, the first 
stage possibly consisting in the incomplete differentiation 
of the tracheides in the region afterwards occupied by the 
leaf-gap ; and the same view is held with regard to the pith l . 
To exclude part of a given tissue as soon as it changes its 
structural nature does not appear to be a morphological 
treatment. 
1 Cf. the case of arrested roots, &c., in Gleichenia (Boodle, ’01 a , p. 732). 
