34 
A. G. Tansley. 
often tends to form a continuous plate of foliage or “ leaf-mosaic.” 
This frond-like structure, in certain species, assumes a gentle curve, 
so that most of its surface is approximately horizontal and receives 
at right angles the relatively weak light reaching the tropical 
forest floor. 
When we turn to the vascular system we find that many of the 
species are what has been called “ polystelic,” and in this case with 
much more justice than in the Ferns. 
In the European species, S. spinosa, the stele is typically 
Lycopodinean and the whole stem is radially organised. In 
5. Mavtensii there is a simple diarch band-shaped stele, the two edges 
of the band being lateral and occupied by protoxylem strands, to 
which the traces from the dorsal and ventral leaves on each side 
are attached. In the larger species with very copious branching of 
the “ frond ” the originally simple stele “ gives off ” similarly shaped 
accessory structures on both dorsal and ventral sides, in somewhat 
the same way that the hollow cylindrical stele of a polycyclic fern 
“gives off” internal accessory strands. As the upper regions of 
the “frond” are approached the “ steles ” diminish in number and 
the ultimate branches contain only a single hand-shaped strand. 
Here then we see an elaboration of the simple stele to meet 
increasing complexity of the assimilating branch-system, just as in 
the Ferns we find elaboration to meet increasing complexity of the 
frond, but carried out on totally different lines. In Selaginella we 
have a very old if not a primitively microphyllous stock which 
modifies whole branch-systems for assimilating purposes. The leaf 
itself is so small as to exercise no influence on the general 
conformation of the vascular system, and corresponds physiologically 
with the ultimate pinnule or segment of the lamina in a fern-frond. 
But the branch-system as a whole retains its plasticity, and 
becomes moulded on lines parallel with those of the fern-frond as a 
whole. 
In some species there is a creeping underground stem or 
rhizome sharply differentiated from the aerial assimilating “ fronds,” 
and here the parallel with a Fern is carried even further. In one 
species, S. Lyallii, this rhizome actually possesses a typical Filicinean 
solenostele, the gaps in which are formed by the departure of aerial 
branch-traces instead of leaf-traces. And further there is an 
internal accessory vascular strand, free in the internodes, but 
coming into connexion with the edges of the branch-gap, so that 
this remarkable form actually imitates the beginnings of typical 
Filicinean polycycly. 
