298 Boiver. — Studies in the Phytogeny of the Filicales. 
bined with the similarity of circular form, the absence of any indusium, the 
approximate uniformity in number of the sporangia, the simultaneity of 
their origin, and the likeness of their relation to the slightly raised receptacle, 
are characters that at once establish a sound basis of comparison between 
the plants named. Moreover, the two species of Gleichenia stand out from 
their genus in the arrangement of their sporangia in the sorus. There is little 
doubt that the ‘ radiate uniseriate ’ type of sorus, as it is seen in G . flabellata^ 
is a primitive one — comparing as it does with what is seen in the Marat- 
tiaceae on the one hand, and in Matonia on the other. Here the centre of 
the sorus is unoccupied by sporangia. But G. linearis has the centre of the 
sorus occupied by one or more sporangia, while in G. pectinata it is habitually 
crowded with them, so that they are flattened against one another. The con- 
stitution of the sorus oi Lophosoria is very like these, while in point of time 
of origin of the sporangia those of each sorus are in both cases simultaneous. 
Both belong technically to the Simplices, while Lophosoria is the only case 
hitherto recorded of this from the Cyatheaceae, which are typically Gradatae. 
6 . Turning to the sporangia themselves, both the Gleicheniaceae and 
Cyatheaceae have an oblique annulus, but they differ in that the dehiscence 
in the former is in the median plane and distal, while in the latter it is 
lateral. It has been pointed out above (p. 275) that G. pectinata with its 
median dehiscence, which requires lateral space to be effective, has in 
its crowded sorus reached fully to the limit of possible efficiency. A modifi- 
cation of its type of sorus or of its sporangium is necessary if any larger 
number of sporangia is to be accommodated, and each is still capable of 
dehiscence. One modification would be the adoption of a lateral dehiscence. 
Another would be an elongation of the receptacle. The best result would 
be by a combination of both. In Lophosoria the lateral dehiscence is 
adopted, but not the elongation of the receptacle. In the rest of the 
Cyatheaceae both modifications are carried into effect, together with 
a basipetal sequence of sporangia, which the elongation of the receptacle 
makes possible. A circumstance which is an indirect support of the view 
that the sorus of Lophosoria is a modification of the Gleicheniaceous type 
is that, though all the sporangia are simultaneous, the lower sporangia 
retain the orientation seen in Gleichenia . This orientation is of no practical 
importance where the sporangia are all simultaneous in origin, and the 
dehiscence is lateral. It may be held as one of those useless survivals 
which are on that account all the more cogent as phyletic evidence. 
From the above considerations it would appear probable that, in the 
production of Lophosoria from some Gleicheniaceous source, a phyletic 
shifting of the point of dehiscence of the sporangium from the distal to 
a lateral position has taken place. This suggestion is supported by the 
fact that the stomium of Lophosoria is not highly differentiated, while there 
is inconstancy in the numbers and relations of the cells which compose it. 
