610 FILICALES 



vascular type to that seen in the more complex Cyatheae, the probable 

 progress has been as suggested in the ontogeny of A. excelsa, while in 

 the sorus the basal indusium appears as a new structure, and the individual 

 sporangia are liable to diminution in size and spore-output, as is exemplified 

 in the extreme form in Cyathea dealbata. Thus there seems good reason 

 to see in the Cyatheae a series having probable genetic relations with the 

 Gleicheniaceae, but advanced on the one hand to the basipetal succession 

 of the sori, and on the other to a high complexity of the vascular system. 



This conclusion is in agreement with the palaeontological facts, for 

 representatives of the Cyatheae have been recognised as present from 

 Jurassic times onwards. It is, moreover, specially interesting to note that 

 the genus Alsophila is among the earliest of the fossils referred with certainty 

 to this family, as exemplified by A. polonica, described by Raciborski 

 from the fire-clay of Krakau. 1 



Of Ferns in which evidence of a basipetal sequence of the sporangia 

 in the sorus has been observed there remain Onoclea, Sphaeropteris, andj 

 Diacalpe, all genera in which the position of the sorus is superficial and 

 the indusium 'basal. The natural place for these genera appears accordingl 

 to be in relation to the Cyatheae. 2 The annulus in these Ferns is almos 

 vertical : in Sphaeropteris it is slightly oblique, and may be traced a 

 continuous past the insertion of the stalk of the sporangium, as i 

 characteristic of the Gradatae ; but in Diacalpe and in Onodea the annulu 

 is interrupted at the insertion of the stalk. These genera appear tc 

 illustrate how, when the basipetal succession is not long continued, an< 

 the orientation of the sporangia not strictly maintained, the annulus is n< 

 longer markedly oblique, but passes over into the vertical position, and maj 

 even be discontinuous at the base. This will be noted also in the 

 Dicksonia-Davallia series. 



SALVINIACEAE. 



It is impossible to leave the Gradatae without mention of the peculia 

 little group of heterosporous water Ferns, of the genera Salvinia anc 

 Azolla. They have been so exhaustively described elsewhere that it wil 

 be unnecessary to give any detailed account of them here, especially a: 

 they are in all probability a side branch from the main series. Examinatior 

 of their sori shows resemblances to the sorus of the Gradatae : it is, however 

 difficult to attach them on these, or on more general grounds to any actua 

 genus of living ferns. It would seem probable that the type from whicl 

 they sprang was homosporous, having an elongated receptacle upon whicl 

 arose a basipetal succession of sporangia, with short thick stalks, and eacl 

 containing 16 spore-mother-cells. That with the differentiation of the spore 

 followed certain other modifications, such as a decrease in number of the femal 

 sporangia, and perhaps an increase of the male sporangia : the former i 



1 Abhandl. Akad. Wiss. Krakau, xviii., 1894. 2 See Studies, iv., p. 55-58. 



