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 Boirychium it is close to the apex (Bruchmann, I.e., Fig. 57). A com- 

 parison of the latter with what has been seen in Psilotum 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 Ophioglossum has the 

 indefinite characters noted above, Botrychium and Helminthostachys show 

 a more definite specialisation 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 Ophioglossum, 

 Psilotum, and Tmesipteris. 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 



