in the ‘ Seedlings' of Certain Leptosporangiate Ferns. 401 
vascular system of an ordinary c polystelic ' type, the idea of a continuously 
branching protostele would never have been formulated. Van Tieghem’s 
work was essentially an attempt to reduce to some common plan the various 
existing arrangements of vascular tissue, and as such deserved, and received, 
the greatest attention. That it has since been shown to be partially 
incorrect 1 in no way alters the fact that it has secured the greatest reward 
which can fall to the work of a pioneer, viz., the stimulation of investigation. 
In 1897 G wynne- Vaughan 2 published his well-known paper on Poly- 
stely in the genus Primula , in which it was conclusively shown that the idea 
of polystely, as advanced by Van Tieghem, must be given up for this 
genus, since the polystelic arrangement does not follow from the branching 
of an originally single vascular rod, but as a result of repeated perforations 
of a vascular tube by gaps occurring in connexion with the leaf-traces. 
This idea was elaborated and extended by Jeffrey in his memoirs of 1897 3 , 
1900 4 , and 1902 5 . According to this writer, there are two types of cauline 
central cylinder : the first, a solid xylem rod surrounded by phloem (proto- 
stele), and the second, the siphonostele, a ring or tube of xylem surrounded 
externally and internally by phloem and endodermis, the whole vascular 
tube being sheathed externally by the cortex, and enclosing a central 
medulla or pith 6 . Of these two types the protostele is regarded as the 
more primitive. The presence of an internal endodermis separating the 
vascular tissue from a non-' stelar ’ pith is an essential point of the theory. 
As is so well known, the interruption of this vascular tube by the exit 
of the leaf-traces, followed later in life by an elaborate overlapping of the 
foliar gaps, results in the polystelic, or preferably dictyostelic 7 , type so 
characteristic of the ferns. 
This is the view which meets with very general acceptance at the 
present day, and the readiness with which its author has extended it to 
all the great classes of vascular plants makes it all the more convincing. 
That, however, the theory is not entirely satisfactory is evidenced by 
the fact that from time to time other views have been put forward, which, 
although essentially based upon the conception of Jeffrey, nevertheless 
differ sufficiently from it as to warrant full consideration. 
The first of these is that put forward by Boodle 8 in his series of papers 
on fern anatomy. This view, defined as briefly as possible, appears to be 
that the whole ring of vascular strands as seen in a transverse section of 
the stem of a dictyostelic leptosporangiate fern, together with the mass 
of parenchyma included within the ring, corresponds to, or is homologous 
with, Van Tieghem’s medullated monostele. It follows that the ‘ pith ’ 
1 Cf. opinion of Schoute, loc. cit. 
2 Polystely in the genus Primula. Annals of Botany, xi. 
8 Trans. Brit. Assn., Toronto, 1897. 4 Trans. Canad. Inst., 1900. 
5 Phil. Trans., 1902. 6 Cf. Faull, loc. cit. 
7 Cf. Brebner, loc. cit. 8 Schizaeaceae. Annals of Botany, xv, p. 404 et seq. 
