Filicales. 
239 
protostele of Lygodium is primitive in the order; that the soleno- 
stelic forms were derived in the course of phylogeny from such a 
type; and that the dictyostelic forms arose from solenostelic types 
by the over-lapping of the leaf-gaps. As regards Schizcea we are 
unable on present evidence to decide whether its stele represents a 
progressive development from that of Lygodium or has been reduced 
from an ectophloic siphonostele. The third view, namely that the 
stele of Schizcea has been reduced from a solenostelic or dictyo¬ 
stelic type, may be dismissed as improbable. 
The gametophyte of Schizaeaceae is generally a flatthallus (11), 
but filamentous prothalli have been reported from Schizcea bifida 
and 5. pusilla (10), (38). Mr. Thomas regards the filamentous 
prothalli as primitive, arguing that we can hardly suppose that their 
structure is due to moisture, since Fern prothalli generally are only 
able to grow under moist conditions (38). Mr. Boodle’s and Dr. 
Jeffrey’s view that the stele of Schizcea is reduced, and Mr. Tansley’s 
and Miss Chick’s that the conducting tissue has decreased, are all 
compatible with the view that the ancestors of Schizcea lived under 
exceptionally moist conditions. But it is not necessary to assume 
that if reduction to a filamentous prothallus has taken place it is 
necessarily correlated with a damp habitat; for if Schizcea bifida is 
reduced, so is S. pusilla, and Mrs. Britten and Miss Taylor found a 
fungus in symbiotic association with the prothallus of this species 
(10). It should be remembered that symbiosis is a condition 
occurring in one of the Hymenophyllaceae possessing a filamentous 
prothallus, and that it is generally held to be one of the factors that 
cause reduction. The view that it has done so in the case of S. pusilla 
is supported by the simple, probably reduced leaves of this species. 
No fungus has been found in symbiotic association with S. bifida, 
but there seems to be at least a possibility that its filamentous 
prothallus, if not due to a moist habitat, may have been inherited 
from a reduced symbiotic ancestor. Another of Mr. Thomas’ 
arguments is that if the retention of small cellular expansions, 
sometimes quoted as evidence for reduction from more normal 
forms, is necessary for the nutrition of Trichomanes, it is hard to 
see why they are absent from Schizcea. But such small expansions 
were described by Mrs. Britton and Miss Taylor in S. pusilla the 
year before Mr. Thomas’ paper was published, and they may well 
have been lost in the hypothetically more reduced 5. bifida. Mr. 
Thomas makes use of another argument open to serious objections. 
He says: “ That the Ferns . . . have been descended from an 
