Bower.—Studies in the Phytogeny of the Filicales. 449 
narrow type of leaf, the sporangia of the same sorus are simultaneous in 
their origin ; there has in fact been a progression from the simple to the 
mixed type of sorus within the genus. A parallel progression is now 
suggested as having resulted in the mixed condition seen in the sorus of 
Plagiogyria ; from a superficial sorus such as that of Gteichenia dichotoma 
or pectinata it would be produced by a slight elongation along the vein, 
together with an irregular interpolation of younger sporangia between those 
first formed, but with the characters of the individual sporangium for the 
most part maintained. Whether or not the Gteichenia type was the actual 
source, it seems probable that there has been here another direct progression 
from a simple sorus to one of a mixed character. The natural affinity 
already recognized by Diels and discussed above, between Plagiogyria and 
the simpler genera of the Pterideae, indicates that this direct progression 
has probably given rise to one of the greatest phyla of Leptosporangiate 
Ferns. The further discussion of this, and the marshalling of the evidence 
to support it, must be deferred for the present, but the facts already in hand 
are sufficient to give the main conclusion a reasonably degree of probability. 
The general effect of such a conclusion will be to condense the brush 
of phyletic lines of the Leptosporangiate Ferns, for, in the case of a very 
considerable sequence of forms with a ‘ mixed ’ sorus, it removes any 
necessity for the idea that they were arrived at through a stage such as 
is seen in the Gradatae, and it refers them in origin directly back to the 
more primitive Simplices. 
Conclusions. 
1. The genus Plagiogyria , merged by Sir W. Hooker in Lomaria , 
is a substantive genus, quite distinct from any other. 
2. It shows its relatively primitive character in the stelar structure, 
the undivided leaf-trace, the simple forked venation, occasional dichotomy 
of the axis, the absence of flattened scales, absence of a ‘ true ’ indusium, 
the sorus initially simple but later showing mixed character, the segmenta¬ 
tion of sporangium, its thick stalk, oblique annulus, and indeterminate 
stomium, and the tetrahedral spores. 
3. It shows resemblances, more or less marked, to all the great series 
of the Simplices, but not to any one of them so clearly as to point to 
close affinity. 
4. On the other hand its characters indicate that it is rightly allied 
with the Pterideae, of which it may be held to be the most primitive type. 
5. Its ‘ mixed ’ character of sorus, without any indication of a gradate 
sequence of sporangia, combined with its primitive characters and its 
probable affinity to the Pterideae, shows that a great phylum of Mixtae has 
probably been derived directly from the Simplices. 
Glasgow, Jan . 31, 1910. 
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