THE FERNS 141 



the sporangium and by its contraction breaks 

 open the wall and scatters the spores. In Osmun- 

 daceae, instead of the ring, there is a little patch of 

 thickened cells on one side of the sporangium. 

 This, as we shall see, marks an old type of spo- 

 rangium, which in the Royal Ferns has come 

 down to our own time. The stem, short, thick, 

 and almost subterranean in our native species, 

 but tall and erect in some Southern forms, has a 

 structure unusual in Ferns, recalling that of a 

 Dicotyledon. There is a ring of vascular strands 

 surrounding a pith; the wood of the strands is 

 separate, but the bast forms a continuous zone 

 all round the outside of the ring. A single strand 

 passes out into the petiole of each leaf. In an 

 American species a little bast is found at certain 

 places on the inner side of the wood, and it is an 

 interesting fact that in a Cretaceous species, 

 from Queen Charlotte Islands, British Columbia, 

 the same peculiarity is found in a more extreme 

 form; here, there is a regular band of bast to the 

 inside as well as the outside of the wood. Thus 

 the Osmundaceous stem seems to have reached 

 its greatest complexity in Cretaceous times. 



Fortunately there is a series of fossils, clearly 

 belonging to this family, which enables us to 

 trace back its history to the Palaeozoic period, as 

 has been done by Kidston and Gwynne-Vaughan. 

 As we go back to the older forms the tendency 



