I IO 
L. A. Boodle. 
nature of internal tissues has been pointed out by Farmer and Hill. 1 
The subject has not been advanced by treating unproved criteria, 
such as continuity and similarity of tissues or the endodermal 
boundary, as final tests of morphology, nor by choosing a morpho¬ 
logical unit and applying it in an arbitrary manner. The morpho¬ 
logy of vascular tissues at present rests chiefly on general theoretical 
considerations, so that what is required is the careful working out 
of further morphological test-cases, if satisfactory ones can be 
obtained, and of phylogenetic series, to provide a firmer basis for 
the theories afloat. As there is rather a lack of such data, we will 
only discuss in a general way the probable mode of differentiation 
of some of the vascular tissues. 
The importance of studying the stele in relation to the leaf- 
gaps has been insisted on by Jeffrey 2 and Gwynne-Vaughan 3 , and 
the vascular structure of the stem is admitted to have been largely 
modified in relation to the leaf-traces. Further it is certainly 
probable that many changes, in the direction of complication in 
stem-structure, began at the nodes and were afterwards continued 
down through the internodes. 
The protostele was probably the type of vascular structure 
possessed by the stem of the more primitive ferns, and may 
be taken as a morphological unit, possibly only a rough one 
(that is to say in a given phylogenetic series the outer limit of the 
stele may perhaps not remain constant). In the protostele the 
centrally placed xylem is generally composed of both tracheides 
and parenchyma (e.g . in GleicJienia). Such parenchyma may be 
localised, as in some Hymenophyllaceae (e.g. Hymenophyllum 
scabrum), where it would be called pith, it it were not that pro- 
toxylem is differentiated at the centre of it. In Hymenophyllum it 
may he taken for granted that the sub-central group of parenchyma 
arose either by multiplication of such parenchyma as is usually 
present in the xylem of a protostele or by the replacement of a 
certain number of tracheides by parenchyma. The same would 
apply to such adventitious medullate roots, as are found in many 
Monocotyledons, where increased diameter of the root is connected 
with their mechanical function as props. Here again the pith is 
probably due to the gradual multiplication of a few parenchymatous 
1 Farmer and Hill. Arrangement and Structure ot Vascular 
Strands in Angiopteris evecta, Annals of Botany, vol. xvi., p. 393. 
2 Jeffrey. The morphology of the Central Cylinder (1900). 
Reprint from Trans. Canadian Inst. 
3 Gwyune-Vauglian. On Loxsoma , Annals of Botany, vol. xv. 
