XXVI] A GENERAL REVIEW OF THE SIMPLICES 233 



recent transitions from a marginal to a superficial position may often be 

 traced: but these do not prejudice the value of the distinction where it 

 appeared in early geological time, and has been tenaciously held since. 



In each of the two principal sequences or phyla of Leptosporangiate 

 Ferns, prefigured respectively by the Schizaeaceae and the Gleicheniaceae, 

 evolutionary advances ma)' be traced involving many of the recognised 

 criteria of comparison : especially those of anatomy, soral constitution 

 individuality and protection, and of sporangial structure and spore-output: 

 while the venation and dermal appendages provide accessory evidence. It 

 will be found that the two main phyla, and certain other subsidiary sequences, 

 run in parallel progression, culminating in types which are characteristic of 

 the majority of Ferns living at the present day. The Schizaeaceae and 

 Gleicheniaceae, like others of the Siinpliccs, have sori inherently limited in 

 their capacity for spore-production. In the Schizaeaceae the marginal 

 sporangia are solitary, and each has been designated a "monangial sorus." 

 The superficial sori of the Gleicheniaceae have at least a plurality of spor- 

 angia: but it has been seen how the construction of the sorus if advanced 

 to its full capacity leads to a mechanical deadlock, as in GkicJienia {Eu- 

 Dicranopteris) pectinata. Moreover in both that inherent weakness of the 

 Simplices remains, viz. the simultaneous drain of all the sporangia for nutri- 

 tion. It will be shown how the nutritional difficulty has been met by the 

 introduction of the Gradate and the Mixed types of sorus in both phyla, 

 together with other modifications. A succession of sporangia is thus pro- 

 vided, so that the drain shall be spread over a longer period. This important 

 change is accompanied by advances in the vegetative system, which also 

 provide material for comparative treatment. Such advances appear to have 

 been initiated early in Mesozoic time. On the other hand, it maybe stated 

 broadly that the Simplices were typically Ferns of the Palaeozoic, though 

 they overlapped into the Mesozoic Period, and some types have even sur- 

 vived to the present day. 



