700 CONCLUSION 



been similar to, though phyletically quite independent of, that in Ophio- 

 glossum ; and the results are, in the former an elongated sorus attached 

 to the leaf-surface, in the latter an elongated sporangiophore which is 

 attached to the sporophyll only at its base. In many other Ferns there 

 is evidence of amplification of the sori, whether by intercalary elongation 

 of the receptacle and a basipetal succession of sporangia, as in the Gradatae, 

 or by marginal extension, as in the Lindsay a- Pteris series, or by superficial 

 spread so as to produce the conditions seen in Gymnogramme, AcrosticJnim, 

 or Platycerium : associated with these is the profuse interpolation of new 

 sporangia characteristic of the Mixtae. It is thus possible to picture how 

 even the most complex and divergent types of spore-production in large- 

 leaved forms may be referred back in their ultimate origin to elementary 

 types, and to recognise how they conform to that general scheme of 

 construction which obtains among the simpler strobiloid Pteridophytes. 



It remains to consider the distribution of the spore-producing members 

 on the plant as a whole. We have recognised the shoot or primitive 

 strobilus as composed of (i) axis, (ii) leaves or bracts, and (iii) spore- 

 producing members. It has also been seen to be probable that originally 

 all the leaves were sporophylls. The primitive shoot appears to have been 

 a general-purposes shoot, in which vegetative and propagative regions were 

 not segregated. But it is evident that two other conditions are possible, 

 that is a shoot bearing (ii) alone, and one bearing (iii) alone ; both of 

 these states are known in living forms, and both may be held to be 

 secondary and derivative. 



The former case, where leaves without spore-producing members are 

 present, is by far the commoner condition of the two, and it appears 

 in the early stage of the ontogeny in almost all Vascular Plants. But it 

 also appears in successive intermediate zones higher up in various plants, 

 and notably in Lycopodium Setago, from which it is called the " Selago " 

 condition (Frontispiece) (Chapter XIII. ). It has been shown that this 

 condition would result from abortion of the spore-producing members, and 

 the fact that this has taken place is clearly indicated by the occurrence 

 of imperfect sporangia about the limits of the region which has remained 

 fertile (p. 162). The converse evidence, that in certain cases (L. Sclavs, 

 Botrychium, and Ophioglossum) the spore-producing members appear very 

 early in the individual life, and that in Lygodiuni sitbalatum the very 

 first leaf may be fertile, further strengthens the view that the whole plant 

 was originally fertile (p. 186), and that the sterile regions, whether basal or 

 intermediate, are so by abortion of the spore-producing members. 



The second case above mentioned, in which spore-producing members 

 are present but no leaves, is less common ; it is seen in Archaeocalamites 

 and in the modern Equisetum. It has been argued at length above 

 (pp. 382-4, and p. 429) that the leaves and sporangiophores in these 

 plants are parts of distinct nature and origin, and that the condition of 

 their strobili is due to abortion of the leaves, of which in Equisetum the 



