152 
A. G. Tansley. 
A solenostele has been defined by Gwynne-Vaughan (’01) as 
“ a single hollow cylinder with phloem and phloeoterma (endodermis 
and pericycle) on either side, the complete continuity of which is 
interrupted only by the departure of the leaf-traces; the gaps thus 
produced being closed up in the internode above before the 
departure of the next leaf-trace.” Sometimes the concavity of the 
trace faces towards the apex of the rhizome (Loxsoma), but more 
often it faces inwards, i.e., towards the dorsiventral plane 
(Hypolepis, etc., and Wibelia, LindsayopsisJ. 
The typical or slightly modified solenostele is rather charac¬ 
teristic of ferns belonging to the middle grades of evolution, though 
it also occurs in several of the lower and a few of the higher types. 
In the case of several solenostelic ferns belonging to the Mixtae, 
there is some evidence that they represent relatively primitive 
types of their groups (Gwynne-Vaughan, ’03). 
The Schiz/EACe/E. 
Before passing on to consider the modifications of the solcno- 
stele and the evolution of dictyostely, we must devote a short time 
to the Schizaeaceae, a remarkable family, in some ways primitive, 
and showing a wide range of vascular structure and some distinct 
peculiarities. 
Of the four genera, the most primitive, Lygodium, is charac¬ 
terised by climbing fronds, often of great length, and with the 
same general construction as those of Gleichenia. The stele of the 
rhizome shows practically the same structure as that of Tricho- 
7 nnnes scandens (Fig. 29), except for the fact that its peripheral 
protoxylem elements are scalariform and not spiral, a fact corre¬ 
lated, no doubt, with the slowness of growth of its rhizome. It 
may probably be regarded as quite a primitive type. 
The petiolar strand of Lygodium is peculiar, and indeed unique 
among living ferns. The whole strand is roundish or oval in 
section. The distribution of xylem and phloem in L. dichotomum 
and L. japonicum may best be made clear by means of a diagram 
(Fig. 47.) cf. Schiztea elegans (Fig. 48). The petiolar strands of 
different species have a general resemblance. In some cases the 
xylem is more rounded in section than in the species figured, and 
in L. palmatum it has the form of an equilateral triangle with a 
protoxylem at each angle, the one situated on the abaxial side being 
distinctly sunk in the metaxylem. Towards the base of the petiole 
the outline of the xylem in L. dichotomum becomes more rounded, 
