Lady Isabel Browne. 
150 
THE PHYLOGENY AND INTER-RELATIONSHIPS OF 
THE PTERIDOPHYTA. 
III.—LYCOPODIALES. 
Lepidodendraceae: AND ISOiSTACEAS. 
The Psilotaceag are now usually placed in a separate phylum, 
the Psilotales, between the Lycopodiales and Sphenophyllales. 
The two genera Psilotum and Tmesipteris are so closely allied that 
it will be unnecessary to discuss their affinity to one another. As 
the phylum contains no other order, the affinity of the Psilotaceae 
will best be considered later in treating of the inter-relationships of 
the phyla. 
The first order of the Lycopodiales to be considered is that of 
the fossil Lepidodendraceae. The anatomy of some of the stems 
of the order is very primitive, for in their primary condition they 
sometimes contained a single, solid, exarch stele, which was 
probably surrounded by phloem, though the latter appears to have 
been sometimes ill-differentiated and by no means typical (19). 
But in most species of Lepidodendron and in all known species of 
the allied Sigillaria a medulla has already appeared; the very 
ancient genus Bothrodendron, also belonging to this order, appears 
likewise to have possessed a pith, but so far the internal structure is 
known in very few examples. There is little reason to doubt the 
primitiveness of the solid protostele in Lepidodendron ; it is also 
clear that the first step in the further evolution of the stele was 
usually the formation of a pith ; this must have been acquired very 
early, though in some cases secondary growth in thickness pre¬ 
ceded the evolution of a pith (9). In the genera Bothrodendron 
and Lepidodendron the ring of wood thus formed appears never to 
have been broken up into separate bundles. Some species of 
Sigillaria retained a continuous ring of xylem, but others show an 
evolutionary advance, for the continuous ring though persisting for 
a considerable distance, is, in parts, broken up into contiguous 
bundles; others again have advanced a step further, for the ring of 
wood is in them broken up into separate, but approximated, 
bundles (14). Thus we see that, as regards the primary anatomy 
of the axis, the Sigillarias are less primitive than, and probably 
derived, from forms of stele such as those of the protostelic 
and siphonostelic species of Lepidodendron and Bothrodendron. 
Secondary xylem is known in nearly all species of any considerable 
