CHAPTER XXXVIII. 



GRADATAE ( Continued}. 



THYRSOPTERIDEAE. 



THE rare monotypic genus Thyrsopteris, which is endemic on the Island of 

 Juan Fernandez, was at once placed with Dicksonta, which appears to be its 

 natural position, though it is better, perhaps, to make it the sole repre- 

 sentative of a separate family. It is a Fern with an upright axis, three to five 

 feet high, covered by the scars of leaves : these have thick stalks, bear 

 a lamina three to four times pinnate : the upper pinnae are sterile and of 

 leathery texture : the lowest pairs of pinnae are fertile but slender : they 

 are as highly branched as the sterile pinnae, but with the surface undeveloped : 

 each pinnule is terminated by a sorus, the whole giving the appearance of 

 a complicated thyrsus. There is some evidence that Ferns of this type 

 existed as early as the Jurassic period. 



The sori have a cup-like basal indusium, surrounding a receptacle which 

 bears numerous sporangia. As in the Hymenophyllaceae, and on the other 

 hand as in Dicksoma, the receptacle is the actual apex or margin of the 

 pinnule ; it appears at first, while the pinna is still tightly coiled, as a smooth 

 cone, slightly flattened in the plane of the leaf. Below this, before the spor- 

 angia make their appearance, the indusium begins to be formed, as a massif- 

 outgrowth : a transverse section at this stage often shows that the indusium 

 is slightly two-lipped, and here we may trace an indication of correspondence 

 with Dicksonia (Cibotium}, or, on the other hand, with Hymenophyllum : 

 but this two-lipped character is only slight, and is not obvious at later 

 stages. The formation of sporangia soon follows, and their succession is 

 basipetal : the first appear at the extreme margin, of which one is shown 

 in Fig. 329 A, the section being perpendicular to the surfaces of the leaf: 

 others then appear in lower positions. The marginal sporangium thus seen 

 is only one of a series which arise along the edge of the flattened receptacle : 

 thus the receptacle is a flattened lobe developed from the margin of the 

 pinnule, as in Dicksonia, while the indusium originates as a growth within 

 the margin, on either side of the pinnule. 



