Bower.—Studies in the Phytogeny of the Fi lie ales. VII. 41 
position in Pterispodophylla resembles that in Pteris elata , in Saccoloma , in 
Acrostichum aureum , and in Thyrsopteris. The marginal series thus serves 
to provide the most striking examples. There are differences of detail 
between all of these cases. It is not probable that any of these Ferns are 
in so close a phyletic relation as to rule out the view that they represent 
independent elaborations under substantially similar conditions. If that 
were their true origin, the differences of detail would be readily understood. 
Sorus of Pteris. 
It has been seen in Histiopteris incisa that the inner (abaxial) indusium is 
absent, and that the reference of the receptacle to a strictly marginal origin 
is dubious. It was suggested that this illustrates the result of a phyletic 
slide of the sorus from the actual margin, as in Lindsay a and Paesia , on to 
the lower surface. In Pteris that superficial origin of the sorus is constant, 
and it has become the generic character. The inner indusium is regularly 
absent, and the persistent upper or outer (adaxial) indusium corresponds 
now not only in appearance, but also in origin, to the margin of the leaf. 
In fact, the phyletic slide of the originally marginal sorus to the surface is 
complete. The sorus thus constituted, covered in by the adaxial indusium 
and served by an underlying vascular commissure, shows in the different 
species a considerable latitude of construction. Selected species illustrating 
this will be described in succession, taking those with the simplest sorus 
first. 
In Pteris longifolia , L., the segmentation of the young pinna is of 
the usual marginal type (Fig. 29, a). The first sporangium makes its 
appearance upon the concave (abaxial) surface, distinctly within the margin 
but near to it (b). The margin itself develops into the indusium, while 
there is no indication of the inner indusium. Later the vascular commis¬ 
sure appears as a compact strand underlying the narrow, slightly convex 
receptacle. The upper indusium becomes much extended as a very 
efficient protective flap. The sorus is mixed, but there are relatively 
few sporangia intermingled with paraphyses. 
Anopteris hexagona (L.), C. Chr. (= Pteris heterophylla , L.), has been 
brought into special prominence by Mettenius 1 in relation to his theory of 
connation ( Verwachsung ) of hairs to form the inner indusium. He states 
for this species that the £ paraphyses ’ are disposed in a series along the inner 
limit of the sorus, and are so closely ranged that they seem to form an 
indusium which grows out distally into hairs projecting as cilia upon its 
margin. His Fig. 17 shows the natural position of these hairs. He 
concludes (p. 7) that the inner indusium ( Schleier ), such as in Pteridium, 
1 Farngattungen, iii, 1 Ueber die mil einem Schleier versehenen Arten von Pteris p. 6. 
Frankfurt, 1858. 
