Bower. — Studies in the Phytogeny of the Filicales. 319 
dally the introduction of the mixed sorus, seem to have given a new phyletic 
impetus to some of the derivative forms, which has eventuated in the great 
Aspidioid sequence. 
Returning lastly to the criteria above cited, we find the following 
progressions illustrated in our series : 
(i) The dichotomous branching is frequent in the Gleicheniaceae, 
but becomes rarer in the higher types. The creeping axis of the earlier 
becomes in certain later types ascending or erect. 
(ii) The peculiarities of the original Gleicheniaceous type of leaf are 
shown in reminiscent details in the Cyatheaceae, but lost elsewhere. 
(iii) There is a progression from primitive hairs to scales, the former 
being characteristic of the lower, while all the higher terms of the series 
show the latter. 
(iv) The vascular system, starting as a protostele in the Mertensia sec- 
tion of Gleichenia , becomes solenostelic in G.pectinata and Lophosoria , but 
is dictyostelic in all the rest. Special accessory strands occur in the 
Cyatheaceae. The foliar trace is correspondingly broken up into separate 
strands in the higher terms of the series. 
(v) The sorus is superficial in all these Ferns, and is not distal on its 
supporting vein (the case of Acrophorus being left in doubt). It shows 
progression from a simple type in Gleichenia and Lophosoria to the gradate 
in Cyatheaceae, Struthiopteris , Onoclea , Woodsia, and Cystopteris. Finally, 
it becomes mixed in Hypoderris , Peranema , and Diacalpe , a condition 
leading probably to that of the Aspidieae. 
(vi) The sporangia are large in the Gleicheniaceae, few in number, 
with large spore-output and median dehiscence. The size and spore- 
output fall in G . pectinata. In Lophosoria it is already of the number 6 4, 
and the dehiscence is lateral. In the Cyatheaceae the spore-number may 
fall to even lower figures, with oblique annulus and lateral dehiscence. But 
finally the annulus becomes almost vertical, and interrupted at the stalk in 
all the highest terms of the series. 
(vii) The antheridium shows reduction in number of the spermatozoids 
which is roughly parallel with the reduction in the spore-number in the 
higher terms of the series. This goes with structural simplification, 
especially seen in the final absence of division in the cap-cell in the most 
advanced types. 
It is believed that the Ferns here treated constitute a true phylum, 
though they probably include a brush of minor phyletic lines. This view 
is based primarily upon the constancy in the superficial position of the 
rounded sorus, borne not distally upon a vein, and the basal insertion 
of the indusium where present ; also upon the characters of the sporangia 
themselves. A substantial parallelism of progression has been traced 
within the phylum in respect of the characters of external form, of dermal 
