152 THE MORPHOLOGY OF PTERIDOPHYTES 



been done for the families and subfamilies selected for 

 detailed treatment, and they have been arranged (Fig. 23) on 

 a circular scheme, according to their advancement index. 

 The most primitive families are near the centre and the most 

 advanced are near the outside. The broken hnes, enclosing 

 'areas of affinity', indicate which groups are most closely 

 related to each other (in the main, the views expressed here 

 accord most closely with those of Holttum*^' ^^). Such a 

 scheme may be thought of as a view, looking down from 

 above, of the 'tree of evolution' of the Filicales, and while 

 it may not be acceptable to all taxonomists, it does avoid the 

 error, which is common to most phylogenetic classifications, 

 of suggesting that one modern family has evolved from 

 another modern family. ^^ 



The two most primitive famihes are the Schizaeaceae and 

 the Gleicheniaceae, and they are also the oldest, being repre- 

 sented in Carboniferous deposits by Senftenbergia and 

 Oligocarpia respectively. Both are represented in the Meso- 

 zoic, too (viz. Klukia and Gleichenites). 



Schizaeaceae 



The Schizaeaceae are represented today by four genera and 

 about 160 species, most of which are tropical or subtropical 

 in distribution. In all of them, the sporangia are borne 

 singly instead of in sori ('monosporangial sori') and they 

 show the most primitive type of dehiscence mechanism 

 known in the FiUcales. In all, the annulus consists merely of 

 a terminal group of thick-walled cells (Figs. 24A-D) and 

 dehiscence is longitudinal. The stalk of the sporangium is 

 short and thick and the spore output from each is 128 or 256. 

 The sporangia arise simultaneously, on the margin of the 

 frond, and are unprotected, except by the inrolUng of the 

 margin, or marginal flaps, of the pinnule. Lygodium is one of 

 the few modern genera of ferns to have fronds of unlimited 

 growth, forming twining structures 30 m or more in length. 

 Unlimited growth is a feature which, in most plants, is taken 



