COMPARATIVE DISCUSSION 485 



part. Such sterilisation, amounting even to the formation of permanent 

 septa, has been shown to take place in other plants, and the theory is 

 not open, therefore, to d priori objection (see Chapters VII. and X.). 

 The frequent absence of sporangia from the tip of the spike is probably 

 due to abortion : they cover the apex in some species, which also show 

 abortion of sporangia lower down (e.g. O. pendulum']. The presence of 

 vascular strands in the septa themselves shows how the physiological supply 

 followed the structural advance : on a theory of lateral fusion of sporangia 

 such a position of the vascular strands would be, to say the least, 

 improbable. Lastly, imperfect or irregular septa have sometimes been 

 seen. Thus the structure, so far as it goes, readily coincides with a 

 theory of extension, and progressive septation to produce the spike of 

 Ophioglossum from some simpler beginning. 



The case of Botrychium is similar, though less obvious, owing to the 

 isolation of the sporangia, while it is complicated by the fact that 

 branching of the spike frequently accompanies septation. That a structure 

 compatible with progressive septation exists is shown by Figs. 253, and 

 its relation to the branching, which brings such conspicuous results in 

 the spike of Botrychium, appears in its simplest form in Figs. 252. It 

 only requires the repetition of the processes, which are thus illustrated 

 in the individual, to lead from the simplest to the most complex spikes 

 in the genus. 



Lastly, in Helminthostachys the ranks of sporangiophores correspond 

 in position to the rows of sporangia in Ophioglossum. An upgrowth from 

 the sporangiogenic band, less regular, but of the same nature as that 

 seen in the branching of the spike of Botrychium, would give the sporangio- 

 phores of Helminthostachys, while the individual development directly 

 represents what this progressive theory demands. This, indeed, is the 

 foundation upon which the present view of elaboration of the spike in 

 the Ophioglossaceae is primarily based : without any preconceptions 

 involving reduction or modification, the theory is founded directly upon 

 the simple facts of individual development. 



The anatomical structure of the shoot in the Ophioglossaceae with 

 its rare dichotomy, which compares rather with the microphyllous than 

 with the megaphyllous Pteridophytes, may next be considered. It has 

 been seen above (p. 464), that the facts observed are compatible with 

 an origin of the axial system from a protostelic state. The stele of the 

 seedling or adventitious bud, is either a protostele or slightly medullated 

 monostele : passing upwards along the shoot there is an amplification of 

 the stele, with swelling of the central pith. In the lower region there is 

 usually a well-marked endodermis : this may be continued throughout 

 the length of the rhizome, but in some cases it fades out upwards, as 

 the stele distends. The xylem in the upper region forms a hollow 

 cylinder or funnel, more or less interrupted by leaf-gaps, where the 

 single strands of the several leaf-traces pass off. The protoxylem is not 



