486 Boodle.—Anatomy of the Hymenopkyllaceae . 
Another case of normal leaves (with stomata) and filmy 
leaves occurring in different specimens of the same species is 
described by Giesenhagen 1 m Asplenium obtusifolium , Linn. He 
comes to the conclusion that in one variety (a) the filmy habit 
is fixed, but that in another (/ 3 ), stomata and intercellular spaces 
occur in the larger specimens, but are sometimes quite absent 
in the smallest. In this species, as in A. resectum , the filmy 
forms are those which grow in the dampest localities. Giesen¬ 
hagen 1 2 regards the simple (filmy) structure of the Hymenophyl- 
laceae as having become hereditarily fixed, as in the variety of 
Asplenium obttisifolium mentioned, the plants being adapted to 
an extremely damp habitat. They have become, as he says, 
practically water-plants, and can absorb water by their leaves, 
so that the conducting elements are naturally reduced. 
In criticizing Prantl’s view that the species of Trichomanes 
with the simplest structure (Prantl’s Hemiphlebieae ) are the 
most primitive, Giesenhagen 3 rightly points out that the 
pseudo-veins which are found in these species are clearly 
reduced veins ; and that therefore the species are reduced ; 
and further that the rarity of roots in several of these species 
must also be regarded as a case of reduction ; because in the 
only species where the embryo has been investigated (although 
they unfortunately do not belong to the Hemiphlebieae) 4 
a primary root is formed. 
It will be as well to recapitulate some of the foregoing. 
Asplenium resectum, &c., show how the filmy structure may be 
derived from normal Fern-structure. * The filmy character, 
leading to semi-aquatic life, will cause reduction in structure. 
Hymenophyllaceae are filmy, and show signs of reduction 
(e.g. pseudo-veins and rarity of roots in some of their simpler 
forms), therefore they are probably reduced from normally 
constructed Ferns, the simplest forms being the most reduced. 
As in Hymenophyllum and Trichomanes , series of reduction 
1 Giesenhagen, Ueber hygrophile Fame, Flora, Erganzungsb., 1892, p. 159. 
2 l.c., p. 174 * 
3 Giesenhagen, Die Hymenophyllaceen, Flora, 1890, p. 437. 
1 Giesenhagen, Flora, 1890, Taf. XVI, Fig. 17, Trichomanes alatum. 
