433 Bower .— Studies in the Phytogeny of the Filicales . 
Sorus and Sporangium. 
The mature sorus and sporangium of Plagiogyria have been described 
by Mettenius ( 1 . c.), and his figures have been quoted by Diels (Engler and 
Prantl, i, 4, p. 281). But hitherto no account has been given of the 
development which would make it possible to enter on an exact comparison 
with other Ferns. The drawing of Mettenius, quoted by Diels as his 
Fig. 149, F, gives a very fair idea of the disposition of the sporangia upon 
the forked veins; this is shown also in our Figs. 1 and 2, respectively for 
P. pycnophylla and P. semicordata , while it is in the former species that the 
development has been traced. Sections of the young pinna show that the 
veins cause a convex swelling of the surface, and it is upon the receptacle 
thus formed that the sporangia make their appearance (Fig. 15, c ). Those 
first formed are almost simultaneous in their origin, but they are followed 
later by others which are interspersed amongst them ; and thus sporangia 
of various ages are in juxtaposition, giving the sorus the well-known 
character of the Mixtae. Fig. 18, c, shows the outline of such a sorus in a 
section traversing it transversely, while the sections shown in Fig. 15, b, 
represent what is seen when cut in a direction following the course of the 
vein : these two latter sections were cut from the same pinna, and a com¬ 
parison of them with Fig. 15, c , shows that there is no fixed succession of the 
sporangia such as is seen in the Gradatae. A comparison of numerous 
sections has disclosed no regular seriation in time of appearance of the 
sporangia; in fact the sorus is a mixed one, but the formation of the 
interpolated sporangia is not long continued. 
The sporangium originates from a single cell, which has a square, 
or more commonly an obliquely wedge-shaped base ; it soon projects con- 
vexly, and segmentation begins by a wall inclined downwards so as to 
impinge upon one of the sloping sides of the wedge-like base (Fig. 16, a) ; 
this is according to the type of segmentation seen in Alsophila and Cyathea 
(see ‘ Land Flora’, p. 637, Fig. 349, c). This wall is succeeded by others 
inclined to the first (Fig. 16, b, c) in the usual manner of Fern-sporangia, 
followed by a cap-cell (Fig. 16, d). A comparison of these drawings, 
together, and with Fig. 16, e, h , shows that there is some variation of detail 
in the basal region of the sporangium, and in the first segmentations; but 
they all result in a relatively massive stalk, which is a natural consequence 
of the deep inclination of the first segment-wall. It may be remarked that 
in the case of the larger sporangium shown in Fig. 16, d , the first segment- 
wall must have impinged directly upon the periclinal basal wall, after 
the manner characteristic of the Schizaeaceae, Thyrsopteris , and Tricho- 
manes ( c Land Flora,’ p. 637, Fig. 349, d). In older sporangia the cells of the 
stalk subdivide so that when mature it is commonly composed of six rows 
of cells in the distal region just below the sporangial head (Fig. 17, b) ; 
