694 G wynne- Vaughan. — Observations on the 
shoot-stele, and stem-stele with each other. In my specimens 
two such strands were usually present. The most constant of 
these was one running from the free margin of the gutter- 
shaped stele of a lateral shoot to that margin of the leaf-trace 
from which the shoot in question arose (a in Figs. 4, 5, and 6). 
The other strand, which was sometimes wanting, ran either 
from the free margin of the gutter of the basiscopic shoot to 
the opposite (acroscopic) margin of the leaf-trace (b in Figs. 
4 and 6), or it started from the free margin of the leaf-gap 
itself and ran to the acroscopic margin of the leaf-trace (c in 
Fig- 5 )- 
A perfect solenostele of almost exactly the same nature as 
that described in the above species of Hypolepis was also found 
in Polypodiu 7 n punctatum . The lateral shoots arose in the 
same way, and, what is even more interesting, the small addi- 
tional strands in the neighbourhood of the leaf-gaps were also 
present. In my specimen they had the same position and 
course as those marked a and c in the diagrams of Hypolepis. 
Several other cases of solenostely were met with in different 
plants, which possess special features of such importance that 
it will be more convenient to deal with them separately 
later on. 
Transitional Types. 
Vascular systems were found in a large number of Ferns 
belonging to several different genera which seem to represent 
a series of stages transitional, or intermediate, between sole- 
nostely and dictyostely. The examination of these forms 
makes it quite clear that the dictyostely of the Cyatheaceae 
and Polypodiaceae is primarily due simply and solely to the 
overlapping of the leaf-gaps in a solenostele ; although it is 
not to be denied that gaps may sometimes occur in the 
vascular cylinder which are not in any way related to the 
insertion of the leaves. It appears also that two different 
factors may be concerned in bringing about this overlapping 
of the leaf-gaps. In the first place, it is evident that if the 
