68o 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 



