274 Bower. — Studies in the Phytogeny of the Filicales . 
between the Mertensia and Eu- Gleichenia types’. The latter are generally 
accepted as being forms showing reduction from the typical Mertensia 
type, as represented by G.flabellata ; they are in fact a side branch. Thus, 
as Boodle points out ( 1 . c., p. 432), Mertensia includes not only the species 
of Gleichenia which is held to be the most primitive anatomically, viz. 
G. fiabellata, but also the most advanced, viz. G. pectinata. The solenostelic 
structure which it shows is to be regarded as derived from a protostelic 
Mertensia type, like that of G. fiabellata. And thus it appears that, putting 
E 2i- Gleichenia aside as a line of xerophytic specialization, Mertensia illus- 
trates a sequence in which G. linearis takes an intermediate place, leading 
from a protostelic to a fully solenostelic structure of the axial stele. It 
will be seen that this runs parallel with certain soral characters. 
The structure of the sorus in certain species of Gleichenia has been 
described elsewhere (Studies, IV. Leptosporangiate Ferns; ‘Land Flora’, 
p. 535) &c.). a nd the conclusion is drawn that the sorus and sporangium of 
G fiabellata are probably the most primitive of the forms examined, a con- 
clusion based partly upon the construction of the sorus, which is radiate and 
uniseriate with about 3-5 sporangia (‘ Land Flora ’, p. 554, Fig. 309), partly 
on the large output of spores from each sporangium. The species of the 
Eu- Gleichenia section show a diminution in number of the sporangia, and 
in their individual output of spores, as compared with G. fiabellata , and 
they present no feature of special interest for our present comparison. But 
the two species which are outstanding from the rest of the genus in other 
respects, viz. G . linearis and G. pectinata , both show indications of advance 
beyond the condition normal for the Mertensia section. This is apparent 
in the larger number of sporangia in the sorus, in their arrangement, and in 
the diminished output of spores per sporangium. The facts have already 
been observed for G. linearis , where the number of sporangia in the sorus 
may be as large as ten, and the central area of the sorus, which is usually 
vacant in other species, is occupied by sporangia (PI. XXX, Fig. C). Hooker 
(‘Species Filicum’, i, p. 13) mentions ‘ capsules 10-12 ’. The sorus being thus 
more crowded than in the usual Mertensia type, the question of dehiscence 
of the sporangia becomes a critical one. In Gleichenia the dehiscence is by 
a split in a median plane of the sporangium, and in order that the opening 
may be effective elbow-room is required right and left of each sporangium, 
and it is therefore only suitable for a laxly arranged sorus. The difficulty 
will be appreciated by examination of the sori in Fig. C, or better of 
the detailed drawing (Fig. D). From this it appears that even in a sorus 
with seven sporangia their close juxtaposition has led to an oblique 
opening of one of them. But the question is not here one of acute difficulty. 
As regards the spore-output the number of spores per sporangium counted 
in specific cases for G. fiabellata were 794, 695, 838, 634, all these figures 
lying between the typical numbers 5 12-1024. For G. linearis the numbers 
