Boodle—Anatomy of the Hymenophyllaceae. 487 
occur which are similar to one another, they appear to be 
parallel genera. As the smaller forms (sub-collateral types) 
are the most reduced in each, the larger (two-banded) forms 
are the more primitive. The species of Trichomanes 
with a solid circular stele (of a type hardly represented in 
Hymenophyllum ) might be regarded as more primitive in the 
series of reduction than T. reniforme and the larger Hymeno- 
phyllums , but many of them have a subcoriaceous frond, so 
they have probably become adapted to life in slightly less 
damp localities than the other species 1 , which would necessi¬ 
tate the differentiation of a stouter stele in equal-sized plants. 
T. radicans , though it is not subcoriaceous, spreads to tem¬ 
perate regions, where the saturation of the air is not likely 
to be of the same degree as in the tropics. Hence the con¬ 
duction from the soil would need to be greater, as evidenced 
by the size of its stele. Its structure points to its having 
been derived from a two-banded type. This species is much 
nearer to T. reniforme in structure than are the other species 
with a solid stele (e.g. T. scandens> T. spicatum ) ; and as T . 
reniforme appears more primitive than T. radicans^ on account 
of sori occurring on most of its veins 2 , it seems probable that 
all the forms with a solid stele owe their type of structure to 
specialization. It has been pointed out that a change to a 
structure like that of T. spicatum might be favoured by a change 
to the upright habit with a short stem, and this would account 
for its being found in T. spicatum and T. Bancroftii. In a 
return to the creeping habit, this structure might be retained 
as in T. heterophyllum , or modified as in T. scan dens. 
Among fossil plants the Botryopterideae have been pointed 
out as showing structural resemblance to the Hymenophyl¬ 
laceae 3 . To quote from the work just mentioned in the note : 
1 Giesenhagen, Flora, 1890, p. 419, ‘ The few Hymenophyllaceae, which live 
outside the primeval forest, are less sensitive to want of water.’ Unfortunately 
there are not sufficient data of the exact habitats of different species. 
2 Prantl, 1 . c., p. 7. 
8 Scott, Studies in Fossil Botany, p. 298. (The author kindly allowed me to 
refer to the proof-sheets of this work.) ‘ The resemblance between the anatomy of 
one of the larger species of Trichomanes and that of 7 ,ygopteris is in fact very 
striking.’ 
