CHAPTER XXXVIII. 
GRADATAE (Continued). 
THY RSOPTERIDEAE. 
THE rare monotypic genus Zhyrsopteris, which is endemic on the Island of 
Juan Fernandez, was at once placed with Dzcksonia, 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 Dyzcksonia, 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 massive 
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. 329A, 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 Dizcksonia, while the indusium originates as a growth within 
the margin, on either side of the pinnule. 
