Boodle.—Anatomy of the Hymenophyllaceae. 483 
become less definite ; and being formed later with regard to 
the region of elongation, it may cease to consist of spiral 
elements. The mass of parenchyma accompanying it may 
be reduced (as it probably has some function connected with 
the typical protoxylem), so that the stele becomes a nearly 
uniform mass of tracheides and parenchyma. This is seen in 
T. spicatum , and as this species has its fertile fronds different 
from its vegetative fronds, some probability is given to the 
view that its structure also is not primitive. T. heterophyllum 
also has dimorphic leaves and is classed next to T. spicatum 
in Hooker and Baker’s Synopsis, but it differs from the former 
species in having a creeping rhizome. In structure, its 
rhizome appears to be almost the same as the stem of T. 
spicatum . These facts probably mean that T. heterophylhim 
is a species that has returned from upright to creeping habit, 
its structure having remained as in T. spicattim. 
T. Prieurii is an upright form with a solid stele in its stem 
containing nearly uniformly arranged tracheides and paren¬ 
chyma. Its protoxylems, however, are spiral and quite 
definite for a short distance after entering from the leaf, but 
as far as could be determined in rather scanty material, they 
gradually die out in passing downwards. Their spiral character 
is probably due to greater elongation of the stem after their 
formation than in T. spicatum. 
To return to the type of T. spicatum , where the first found 
elements of the stele are scattered, some perhaps represent 
spiral protoxylem of an earlier type of plant, others probably 
do not, and under new conditions of growth a new protoxylem 
of spiral elements might arise in a different region of the stele. 
This is the theory put forward to explain the structure of 
the stele in T. scandens. The stele here is of the solid kind 
found in upright species like T. spicatum , but there is a 
definite peripheral spiral protoxylem. This species may be 
regarded as having perhaps given up an upright habit for its 
present creeping one, and developed a new peripheral proto¬ 
xylem of spiral elements on account of its more rapid growth 
and greater elongation of its internodes. 
K k % 
