362 Bower — The comparative examination of the 
stalk, and this character is foreshadowed by the first segmen- 
tations. In all the plants hitherto cited the wall of the 
sporangium consists of a single layer of cells, while a tapetum 
of not more than two layers intervenes between it and the 
spore-mother-cells. 
Taking next the Osmundaceae, here as in the Schizaeaceae 
the sporangia are of relatively large size, and have short and 
thick stalks, and so they approach the Marattiaceae in these 
respects. The origin of the sporangium has been investigated 
by Prantl 1 , and by Goebel 2 : the latter leaves the point still 
open whether or not the sporangium of Osmunda be referable 
in its origin to a single cell, as in the true leptosporangiate 
Ferns ; and this is just one of those questions which, when 
dealing with transitional forms is so difficult to answer with 
certainty. If the origin be traced far enough back, doubtless 
the whole sporangium could be referred to a single cell, just 
as the whole plant may be ultimately referred in origin to the 
ovum ; but without straining the point in such a way as that, 
it is often difficult to trace back the sporangium of Osmunda 
to a single cell, and in any case, a single initial cell does not 
project obviously outwards before segmentation, as is the case 
in the true leptosporangiate Ferns. 
In Osmunda regalis the sporangia first appear as rather 
broad and very slightly convex projections on the wings of 
the pinnae and pinnules (Fig. 74 ) ; more than a single cell 
may take part, and though such cells often bear an obvious 
relation to one another, still it cannot be said in the same 
sense as in the leptosporangiate Ferns that the sporangium 
results from the outgrowth of a single superficial cell. In all 
cases, however, one cell distinctly takes the lead, and this we 
may call the initial cell (Fig. 75) ; but the arrangement of its 
division-walls does not, as in the true leptosporangiate Ferns, 
conform to any strict plan ; the initial cells are oblong as seen 
in vertical section, and the first divisions are longitudinal, so 
as to meet the basal wall (Fig. 74 ) : both in the segment thus 
cut off and in the central cell, periclinal, or sometimes oblique 
1 Bot. Zeit., 1877, p. 64. 2 Schenk’s Handbuch, III, p. 387. 
