CHAPTER XXIX 



DICKSONIACEAE 

 /. Thyrsopterideae 



This sub-family is now represented only by the genus Thyrsopteris including 

 the single species T. elegans Kze, endemic in Juan Fernandez: but many 

 authors have shown evidence that Ferns of this type existed in the Jurassic 

 Period (Seward, Jurassic Flora, Vol. I, p. 98, etc.). The isolation of this single 

 living species suggests that it is an ancient survival. It is a Fern with an up- 

 right axis, sometimes as thick as a man's thigh, and three to five feet high. 

 It spreads by runners, from which shoots come up at some distance from the 

 parent plant. The leaves have thick stalks, and are 3-4 times pinnate: the 

 upper pinnae are sterile and leathery, the lowest pairs are slender and fertile 

 (Fig. 527). Each pinnule of these terminates in a sorus, the whole giving the 

 appearance of a complicated thyrsus. The dermal appendages are mucilagi- 

 nous hairs, apparently unbranched : on the rhizome there are also stiff brown 

 bristles, but there are no scales. The thin runners are solenostelic : but where 

 larger, a single strand or sometimes more may be found in the pith. In a 

 larger stem this has been found to expand, as in Dennstaedtia, into a medul- 

 lary system with a compensation-strand filling each leaf-gap (Fig. 528). It 

 would be interesting to see the structure of one of the largest stems, for it 

 might well show a structure not unlike some " Psaronieae." The leaf-trace 

 passes off as a corrugated horse-shoe : in larger leaves it becomes more 

 corrugated higher up, and divides opposite the lateral pneumatodes into 

 three straps, still preserving the horse-shoe form (see Vol. i. Fig. 161, 5). 

 These characters accord with what is seen in some Dennstaedtiinae. 



The sori are marginal in origin, and they retain that position. Each has 

 a cup-like indusium surrounding the receptacle, which bears a basipetal 

 sequence of sporangia (Fig. 527, Z>, £). The indusium which originates as 

 a massive superficial outgrowth on each leaf-surface, below the marginal 

 receptacle, is at first slightly two-lipped, but this is not obvious in later stages. 

 The formation of sporangia soon follows, the first appearing on the extreme 

 margin (Fig. 529): but the receptacle is really a flattened lobe, and a series 

 of sporangia occupies its margin as in Schizaea: these are followed by 

 others in gradate sequence. 



The sporangia are large with a short massive stalk, which shows about 

 eight cells in transverse section. The head bears an annulus with 50 or more 

 cells forming a curiously twisted hoop (Fig. 529, E, F). Rather more than 

 half of the cells are indurated, while the rest form an obliquely lateral 

 stomium, the structure of which is complex, and not strictly defined. Rup- 

 ture, the exact position of which may vary, is lateral. The typical number 



