724 Gwy nne- Vaughan. — Observations on the 
and Davaltia speluncae (Fig. 26), &c., the abaxial curve forms 
practically the whole of the strand, the adaxial arcs being 
reduced to small inflected hooks at the ends of the arms. In 
view of this suggestion it is interesting to find that whenever 
a petiole branches a vascular strand is always given off to the 
branch from the point where the adaxial arc or the xylem 
hook joins on to the abaxial curve. In many of the cases 
examined a single strand alone passes into the petiolar branch, 
but in others a second strand is also present, which invariably 
departs from the abaxial curve itself at the point where it 
is folded inwards ( x in Fig. 15). 
Lateral Shoots. 
In most of the solenostelic and in many dictyostelic Ferns 
lateral shoots are frequently to be found growing out from the 
bases of the petioles. Their vascular systems are usually con- 
nected up with the adaxial margins of the leaf-trace, rarely, as 
in Pteris elata and Dicksonia Barometz , with the middle of its 
abaxial curve. The vascular system at the base of the lateral 
shoot is often a completely closed cylinder, even when the 
main axis is perfectly dictyostelic, e. g. Adiantum trapezi- 
forme. Sometimes a single central cylinder with a solid mass 
of xylem is present. As this stele passes through the cortex 
of the main axis a core of phloem appears in the centre of the 
xylem, then a solenostele is formed, and finally it may become 
more or less dictyostelic, as in Dicksonia Barometz. A simi- 
lar structure is also to be found in the lateral shoots of 
Dicksonia adiantoides , but here the change from the solid 
central cylinder into the solenostele takes place very rapidly, 
and the solenostele is afterwards permanent. In this plant, 
and also in Pteris elata , the internal vascular strands do not 
occur in the lower part of the lateral shoots. In Dicksonia 
rubiginosa , however, they are usually present, even at the 
very base. So far as my investigations went on this point 
they may be summed up generally by stating that the ontogeny 
of the vascular system of the plant as a whole is very fre- 
