MARATTIACEAE 



[CH. 



sori with circular outline like those of Christensenia appear isolated upon the 

 enlarged surface (Fig. 403, a, b, c). It seems probable that the state seen in 

 Christensenia was thus acquired during the descent of a gradually broadening 

 leaf. The frequent occurrence of soral fissions on its leaves supports this 

 view (Fig. 403, lower series). Thus this broadest-leaved living genus with 

 its scattered sori may be brought into line with the rest, but as a derivative 

 type. 



® d^ii 



F'g- 403' ^1 I'' ^' Danaea alata Smith. a = a fertile pinna with 

 many normal sori : the arrow indicates an abnormal fission, b, c 

 show more numerous fissions resulting in irregularly formed sori, 

 distributed over a slightly enlarged surface ( x 2). The sori shown 

 below {a-e) are from Christensenia, and they show states of 

 partial or complete abstriction. 



The parallel between the synangial sori and those with the sporangia 

 separate raises the question of the probable evolutionary relations between 

 these types. It is significant that in the earliest fossil Ferns the sporangia 

 are usually separate {Stanropteris, Botiyopteris, Zygopieris) : but early 

 synangial sori are usually present also {Ptychocarpus), while in some 

 examples the sporangial fusion may be probable though less certain. Scott 

 remarks {Studies, Vol. i, p. 366) that "the fossil data suggest, on the whole, 

 that free sporangia represent the original form of Filicinean fructification, 

 and that their cohesion to form synangia was a secondary modification, 

 though one which, in certain groups, took place at a very early period." The 

 evidence from living plants supports this view. The comparisons already 

 drawn for the living Ophioglossaceae make it appear probable that in them 

 evolution has led from the state of separate sporangia of Botrychiuni and 

 HelmintJwstacJiys to the synangial state of OpJiioglossnni. Similarly, though 

 with less cogent evidence, it may be held that the synangial state of JMarattia^ 

 Protomarattia, and Danaea, a series parallel in soral form to that oi Angio- 

 pteris, Archangiopteris, and Danaeopsis, has also resulted from a secondary 

 fusion of their sporangia to form synangia. 



As the development of the individual sporangium has been found to be 

 essentially the same in the several genera, it will suffice to describe it for one 

 only, and Angiopteris may be selected as being the most familiar. At an 



