XVIII] COMPARISON 39 



Vaughan point, however, to the great similarity of the traces of Dineiiron 

 (Fig. 325, 14), and of Clepsydropsis (Fig. 325, 7), and they suggest that their 

 assumed ancestral form (Fig. 325, 6) was really the common starting-point 

 for both groups. This appears to be a probable conclusion : both types are 

 of early occurrence, Clepsydropsis from the Upper Devonian and Dinenron 

 from the Culm. The difficulty that the more complex types of stem- 

 structure are found among the biseriate Zygopterids need not be held as a 

 serious obstacle. If as above suggested the stellate structure of the stele is 

 a function of size, any Clepsydroid stem that increased in bulk might be 

 expected to acquire it: and the more advanced stelar structure seen in 

 either Asteropteris or Asterochlaena might be associated with a simple leaf- 

 trace if the leaf did not increase in proportion. Accordingly it seems not 

 improbable that from some simpler source two parallel sequences arose, the 

 Clepsydroideae and the Dineuroideae. But what the actual relation of 

 Staiiropieris was to these must remain uncertain : though the relatively 

 small size of its petiole and conducting tract, and the character of its 

 sporangia, would indicate for it a relatively primitive position, related per- 

 haps to Dinenron. 



Those sporangia of the Coenopteridaceae which are sufficiently well 

 known as regards structure and relation to the plants that bore them, present 

 features interesting for comparison. They are all of the Eusporangiate 

 type, with thick stalks and walls, a large spore-output of homosporous 

 spores, and simple mechanical arrangements for dehiscence but not for 

 ejection of the spores. They may be seriated according to their position 

 and relations so as to illustrate steps which may well have occurred in the 

 evolutionary origin of a simple uniseriate sorus, so prevalent among primi- 

 tive Ferns. But in placing them in this sequence it must not be assumed 

 that the examples actually formed a phyletic series: their seriation can 

 only be used here for the purpose of illustrating a probable method of origin 

 of a familar type of sorus. 



A primitive state is that of Stanropteris, with solitary distal sporangia 

 borne on the ultimate branchlets of a much-divided leaf. This position 

 corresponds to that seen in the Psilophytales. If the pedicels were abbrevi- 

 ated and crowded together the condition seen in Etapteris would result, the 

 position being still distal, with the individual sporangia facing inwards. 

 Further compacting of such a group would lead to the sporangia being 

 seated on a common pedicel, again facing inwards, as in Corynepteris: but 

 here the position is no longer distal but lateral. A further step illustrated 

 in Anachoropteris gives synangial fusion of the sorus, which was here borne 

 on the incurved margin of the small pinnules. A "phyletic slide" of the 

 sorus to the lower surface of a widening leaf-segment, such as is seen in 

 many phyla of living Ferns (Chapter XIl), would then give a soral state similar 

 in structure and position to that seen in the Marattiaceae. 



