592 FILICALES 



of the sporangia are intermixed. The probable position of these genera 

 will be considered later. There remains, then, only the old comprehensive 

 genus Dicksonia. This was divided in the Synopsis Filicum into three 

 sections Cibotium, Eudicksonia, and Patania ( = Dennstaedtid). While 

 Cibotium and Eudicksonia have obvious relations to Thyrsopteris, Patania 

 (Dennstaedtid) clearly approaches the genus Davallia, and especially to that 

 section of the old genus which has been separated as the independent genus 

 Microlepia : these relationships will now receive the support of develop- 

 mental and anatomical evidence. I shall follow Prantl 1 in separating 

 Dennstaedtia and Microlepia from the position given them in the Synopsis 

 Filicum, and recognise them as constituting, perhaps with certain other genera, 

 a natural sub-tribe under the name of the Dennstaedtiinae Prantl, having a 

 position between Dicksonia and Davallia, and constituting with them a 

 natural sequence. It will be shown that following this series from Thyrso- 

 pteris to Davallia we shall pass from a type with basipetal sequence of the 

 large, short-stalked sporangia, with oblique annulus, to forms with a mixed 

 sorus, smaller, long-stalked sporangia, and a vertical annulus. The receptacle, 

 which is a prominent feature in the former, is reduced, or represented only 

 by a flat surface in the latter types. The gradual nature of these parallel 

 steps seems to indicate that the whole series is one of natural affinity, as 

 indeed has always been recognised by systematic writers. 



DICKSONIA (EXCL. Patania}. 



The genus Dicksonia includes some large Tree Ferns, and others of smaller 

 stature but with prevalent radial construction. Some of the smaller species 

 closely resemble small plants of Thyrsopteris : like it they have leaves 

 repeatedly pinnate, with numerous sori borne at the margins, but without 

 any differentiation of sterile and fertile pinnae. The sori are protected by 

 a two-lipped indusium, but the lips are unequal, and their character has 

 been used as a basis of division of the genus. It will be shown that the 

 receptacle itself is marginal, and that the lips of the indusium are develop- 

 mentally outgrowths from the surface of the pinnule, just as in the 

 Hymenophyllaceae and in Thyrsopteris. Thus there is essential corre- 

 spondence with these Ferns, and the differences are rather of habit and 

 size than of the more fundamental features of the sorus. 



SPORE-PRODUCING MEMBERS. 



The sorus in this genus has already been investigated by Gliick, 2 

 who points out that the receptacle arises from the original leaf-margin, 

 while the two lips of the indusium spring from the upper and lower 

 leaf-surfaces. The structure of the young receptacle, as seen in Dicksonia 

 (Cibotium) Schiedei, Baker, is like that of a leaf-margin, with a marginal 



1 Arb. Konigl. Bot. Cart, zu Breslatt, 1892, p. 18. 2 Flora, 1895, Heft 2. 



