THE SPORE-PRODUCING MEMBERS 699 



seen in Ferns is similar in kind to that of strobiloid types, but modified 

 in accordance with the great amplification of the sporophyll, with its 

 continued apical growth and often profuse branching : this was accom- 

 panied by increase in number of the sporangiophores (sori), fission being 

 one prominent source of that increase, and also by a tendency for the 

 sorus to diffuse itself as scattered sporangia over the enlarged surface, 

 producing thus the non-soral state as a secondary condition : moreover, 

 the position of the sori shows frequent tendency towards the lower leaf- 

 surface. From this point of view the Fern-type does not stand apart 

 from the rest in the essentials of its morphology, but only in the fact 

 that it has proceeded to a larger-leaved state, and that this has brought 

 with it secondary changes of the spore-producing members. 



There is a considerable bulk of evidence to show that, apart from 

 fissions, the sorus or sporangiophore has also been capable of extension 

 in the course of descent : this is to be found in certain points of structure 

 which have not been satisfactorily accounted for on any other footing. 

 It has been noted that in the fossil Equisetales the number of sporangia 

 on each sporangiophore is commonly four (p. 425), but that modern 

 Equiseta have usually more. In the Psilotaceae and Sphenophylleae the 

 number may be from one to six, the lowest number being probably in 

 some cases due to reduction : thus fluctuating numbers are a common 

 feature in the simpler types. In the Ophioglossaceae the fluctuations are 

 within wider limits, and the larger numbers of sporangia are associated 

 with an apical growth of the sporangiophore, which is either of very short 

 duration or entirely absent in other cases. The result is in Ophioglossum 

 the elongated spike, with its lateral rows of sporangia partitioned some- 

 times imperfectly from one another (Fig. 361 L). The structure bespeaks 

 a progressive condition in which septation has played a leading part 

 (p. 404). In Botrychium, profuse branching parallel to that of the 

 sterile leaf, occurred, and it is very closely related with septation of the 

 individually projecting sporangia (p. 454) ; lastly, in Helminthostachys 

 the rows of sporangia of Ophioglossum are replaced by dense ranks of 

 sporangiophores (pp. 455, 485), and their origin is believed to have been 

 virtually a repetition of that process of septation and upgrowth above 

 recognised in the origin of the sporangiophore from a simple sporangium 

 (Fig. 361 N). All these amplifications of the sporangiophore are consistent 

 with physiological probability, as shown in Chapter XXXI. 



In the Ferns also similar extension of the sporangiophore (or sorus) 

 is seen, but it has taken a different form in accordance with the expansion 

 of the leaf-surface to which it remains attached. It is exemplified in the 

 simplest form in the Marattiaceae, in which the structural condition of 

 Danaea seems plainly to be the result of elongation and progressive septation 

 of a sorus of the same type as that of Marattia (Fig. 278 c, E) ; the 

 partial septations are themselves specially convincing evidence of how the 

 highly septate state has been acquired (p. 518). The progression has 



