372 Bower. — Studies in the Phytogeny of the Fiticales. 
formed beneath the continuous receptacle (Fig. 3,^*; compare also Text-fig. 
14, p. 400). But it is important to note that this is not constant. Sections 
may be found in series cut from a fertile pinna in which both sorus and 
commissure are interrupted. Such a section is shown in Fig. 3, h , and such 
cases are important for comparison with Plagiogyria and Matteuccia, in 
neither of which are the sori or the commissures continuous. 
In B. discolor the origin of the wing of the pinna is, as usual, from 
segmentation of the marginal series, but it is rather more robust than in 
Plagiogyria. It is seen in an early stage in Fig. 3, a , where clearly the 
whole is referable in origin to a normal segmentation. Already extra 
periclinal divisions may be seen at x , which foreshadow the thickening later 
formed at the angle of curvature of the transverse section (compare Fig. 3, 
g, h), and constitute those ridges which are a marked external feature in the 
mature state. Such ridges initiated early, with extra periclinal divisions 
as a frequent feature, are common in the genus ; they are, however, 
variable in their position, size, and mature structure ; they may even vary 
at different levels on the same fertile pinna, as may be seen on examination 
of the pinna from without, as well as from serial sections through it 
(compare PI. XXIII, Fig. 5 of B. lance olatuni). Up to the condition shown 
in Fig. 3, a , there is no definite sign of initiation of the ‘ indusial ’ margin. 
The interest will naturally centre round the origin of those protective flaps 
which are so prominent in older stages (Fig. 3,/,^, h). Their position is 
apparently marginal, as in Plagiogyria , but their actual relation to the mar- 
ginal initial is rather indeterminate. As the marginal segmentation proceeds, 
the marginal cells become smaller (Fig. 3, b, c , d , e) t and may finally divide 
up as in Fig. 3, c. As a rule in this species, it appears that the marginal cell 
does not itself directly furnish the indusial growth, but at an early stage 
divisions begin in an adjoining segment (Fig. 3, b } c, d , e), accompanied by 
more rapid growth, so that the flap (i) begins even at this early stage to 
project. It should be remarked that the sections drawn have been selected 
from a very large number examined, and are believed to represent typical 
examples. But there is a good deal of variety of detail in nearness of 
origin to, or even coincidence of the indusium with the marginal cell itself. 
Such conditions as those shown suggest the very first stages of a ‘ phyletic 
slide 5 of the indusium, or phyletic margin, from a strictly marginal position 
as it is in Plagiogyria , to a superficial position. That such a c slide * has 
taken place is, in point of fact, the working hypothesis which will accompany 
the description in this and other species. 
The cells which will give rise to the sorus itself have meanwhile 
become recognizable, by their deeper prismatic form (Fig. 3, c) ; they begin 
to project (Fig. 3, d , e), and commonly form a group showing the features 
of a slightly basipetal sorus, those at the centre being more advanced than 
those on either side. The position of the sorus is very near to the indusium 
