Filicales. 
^35 
is probably correct, being supported by the fact that the sporophyte 
is clearly reduced, and that the gametophyte has, as pointed out by 
Professor Bower, been exposed to the same conditions. Further, if 
the filamentous type were primitive we should expect to find it 
retained by primitive members of other filicinean orders. This 
latter argument is slightly weakened by the fact that filamentous 
prothalli have been found in two species of ScJiizcea (10), (38). 
As regards the affinities of the Hymenophyllaceze with the 
Botryopterideas the two orders have much in common. This is 
true whether we select as primitive the forms with a solid exarch 
stele, or, in agreement with Mr. Boodle and Mr. Tansley, regard the 
endarch forms of both orders as primitive. In the former case 
there is a distinct similarity between the steles of Tubicaulis and 
Trichoiuanes scandens. In the latter case we have, as remarked by 
Mr. Boodle, an interesting likeness between Zygopteris and such a 
form as Trichomanes reniforme ; in both we have small internal 
tracheides embedded in parenchyma and surrounded by vascular 
elements. Tbe present writer holds that these structures arose in 
a different way : that the internal tracheides of Zygopteris represent 
the remains of centripetal xylem, and that its outer xylem was 
probably centrifugal; whereas Trichomanes reniforme arose (as the 
writer believes) by reduction from a form with a solid stele, its 
exarch ancestors having lost their protoxylem, which afterward 
reappeared in the centre of the stele. But it is possible, and many 
will think it probable, that both orders were primitively endarch. 
Then the resemblance of species of Trichomanes to Zygopteris is 
striking; it extends to the presence in many species of both 
genera, of an axillary shoot, whose vascular bundle is in both cases 
cut off from the leaf-trace in essentially the same way. The rarity 
of the axillary shoot makes it a character of importance; but it 
has been suggested by Mr. Boodle that this mode of branching 
may be a modified dichotomy, the undivided trace (as the trace is 
termed before giving off the bundle of the axillary shoot) repre¬ 
senting the weaker branch of the dichotomy, that may be supposed 
to bear the apparently subtending leaf. If the axillary shoot with 
its leaf represents a modification of dichotomy, it may have arisen 
independently in the two orders, and not be proof of close affiinity. 
Thus whether the endarch or exarch Hymenophyllaceas are primitive, 
both types of stele are derivable from the Botryopteridese. Further, 
Mr. Tansley has pointed out how the Hymenophyllaceous petiolar 
bundle, which may be from the first an open C, or may be given off 
