680 CONCLUSION 
Such examples, showing a parallelism of progression raise the question 
whether in the Pteridophytes generally the apical segmentation with a 
small-celled meristem and several initials was not the more primitive state, 
and that with the more definite segmentation of a single initial the derivative. 
It is not possible in the present state of knowledge to come to a definite 
conclusion on this point ; and in the sporangiophoric Pteridophytes, including 
also the Ophioglossales, the evidence is less clear than in the cases above 
quoted ; for in them there is a prevalence in the very isolated living genera 
of a definite segmentation with a single initial: there is, it is true, nothing 
to preclude the view that they also were derived from forms with several 
initials: certainly their eusporangiate sporangia, and deeply sunk antheridia 
and archegonia, which usually go with the less precise segmentation at 
the apex, would suggest that this was so. 
Closely associated with the continued apical growth of the shoot is 
the formation of the appendages, leaves, emergences, and hairs. The 
leading fact with regard to the leaves is that in all cases they are found 
to originate normally in the same way, by enation from the pre-existent 
axis, and in acropetal. succession: the embryological comparison given 
above shows that this holds even for the protophylls. It applies equally 
for the small leaves of the strobiloid types and for the larger and more 
complex leaves of the Ophioglossales and Filicales: moreover the leaves 
are dorsiventral, and show a constant orientation to the axis which bears 
them. The high degree of persistence of their relation to the axis, 
notwithstanding the differences in size, form, and number, indicates that 
the parts large or small are substantially of similar nature throughout the 
Pteridophytes, though not necessarily homogenetic. The discussion in 
Chapter XI. has led to the conclusion that the leaves originated in descent 
as they are seen to do now in the normal course in all Pteridophytes, 
viz. by enation from the apically growing axis. It is held as probable 
that the process of leaf-formation which appears in every normal ontogeny, 
should represent the mode of their phyletic origin. 
The view that there is an inherent improbability in this mode of 
phyletic origin of the leaves has already been alluded to (p. 659). 
But leaves are not the only appendages of the simple shoot: emergences 
and hairs must also be considered, and from these some light 
may be derived as to the origin of appendages at large which may 
illuminate the probable origin of leaves. Emergences occur in isolated 
genera and species of Vascular Plants, both in Pteridophytes, and Seed- 
Plants: they sometimes contain vascular tissue, and in early stages of 
development may closely resemble leaves. Both emergences and hairs 
arise ontogenetically by enation from the plant-surface, and both are often 
irregular in their position. Is there any reason to believe that these 
sporadic appendages of the shoot were fashioned out of some pre-existing 
organ.? ,The very irregularity of their. position in the individual, and 
of their occurrence in the race precludes such a view for them: the conclusion 
