484 OPHIOGLOSSALES 
the sporangiophores in the more isolated Cheirostrobus point to a similar 
origin of its very complex state. It would thus appear probable that the 
sporangiophore of the Sphenophyllales and the spike of the Ophioglossales 
are parts not only similar in position and function, but also susceptible 
of chorisis. This may be held by some to be only a distant analogy: 
but such analogies have a way of developing into convincing evidence when 
they prove to be cumulative. 
The first appearance of the spike in Ophioglossum is upon the upper 
surface of the sporophyll, in a median position some distance from the apex, 
but in Botrychium it is close to the apex (Bruchmann, Zc., Fig. 57). A com- 
parison of the latter with what has been seen. in Psz/otum and Tmesipteris 
(Figs. 230, 232), shows a striking similarity in the position of the spore- 
bearing parts relatively to the apex of the sporophyll. If this similarity 
also be only one of analogy, it is at least a singularly close one. In the 
Psilotaceae only two lateral leaf-lobes are subsequently formed, but in 
Botrychium a considerable number. The pronounced apical growth of the 
Ophioglossaceous spike is also a point of difference from the sporangiophore 
of the Psilotaceae; but it naturally accords with the more elongated form 
when mature. 
The details of development of the sporangium afford. material for 
further comparison. It has been shown how in the Psilotaceae there is 
special difficulty in recognising the exact limits of the sporogenous masses 
at an early stage of the sporangium, and that there is no definite 
tapetum. In Ophioglossum the condition is similar: there is the same 
indefiniteness of the sporogenous masses when young, and the same 
absence of a definite tapetum. On the other hand, the Ophioglossaceae 
themselves show interesting gradations: for while Opfzoglossum has the 
indefinite characters noted above, Botrychium and Helminthostachys show 
a more definite specfalisation of the sporangia, which goes along with 
their smaller size; for here the tapetum is a definite one, and originates 
outside the sporogenous tissue. There is thus an approach in the 
Ophioglossaceae to the condition seen in the Eusporangiate Ferns. The 
indefinite condition of the sporangium is exceptional among the Pterido- 
phytes: of living forms it is most prominently seen in Ophzoglossum, 
Psilotum, and Tmestpteris. This similarity materially strengthens the 
comparison between the spore-producing members of the Psilotaceae and 
Ophioglossaceae. 
From the development of the individual spikes of the latter some idea may 
be formed of the steps which probably led from the simple structure on which 
the comparison has so far rested to the more complex. In Ophioglossum 
the lateral rows of sporangia arise from the sporangiogenic band : its cells, 
originally alike, differentiate into sporangial wall, septa, and sporogenous 
groups: in point of origin the latter are all alike, and the structural details 
are in accord with a theory of progressive septation, that is, a conversion 
of part of the potentially fertile tissue into sterile septum in the enlarging 
