214 GLEICHENIACEAE [CH. 



Comparison 



While the Schizaeaceae with their four Hving genera are marked by the 

 wide range of their characteristics, both formal and anatomical, the Gleicheni- 

 aceae are essentially a monotypic family: indeed at one time or another all 

 the living representatives have been ranked under the single genus GleicJienia 

 Nevertheless the various living species show variations both in their anato- 

 mical and their soral characters that provoke phyletic comparison: and 

 this is all the more pointed since those differences appear in plants so 

 clearly akin to one another. At the same time there can be no doubt of the 

 relatively primitive character of the Family as a whole. This follows not 

 only from the soral features, but also from the general morphology and 

 particularly from the anatomy: with this also the geological evidence is in 

 full accord. 



The prevalence of equal dichotomy in the usually extended rhizome is 

 significant for the genus GleicJienia, and it appears also to dominate the 

 development of the upward-growing shrubby branches of Stromatopteris, 

 though it is still uncertain "what is the relation of these branches to the 

 underground rhizome. In Platysovia a dichotomous branching of the main 

 axis appears occasionally. A general comparison of the shoot of GleicJienia 

 with the dichotomous rhizome of Lygodium appears justified, while the 

 form of the shoot of PJatyzoina with its crowded leaves has features in 

 common with the xerophytic MoJiria. The leaves throughout the Family 

 are based upon sympodial dichotomy: but it may run in GleicJienia to 

 an unusual and very characteristic development, which depends upon a 

 periodic interruption of the apical growth. Branching may be rare or absent, 

 as it is in Stromatopteris and Platyzoma, and a like state is seen in the 

 small Andean species G. simplex, which appears to retain permanently the 

 type of leaf characteristic of the sporeling. Such plants serve to link the 

 peculiar leaves of the larger Gleichenias with ordinary leaf-development as 

 seen in primitive Ferns. The larger types are comparable when fully 

 analysed with what is seen in Lygodium, allowance being made for difference 

 of proportion of the parts, for the interruptions of apical growth in GleicJienia, 

 and for the cessation of apical growth in the pinnae, which arrives earlier in 

 Lygodium than in GleicJienia. There are no basal or stipular developments 

 in either of these Families, such as characterise so many of the most primi- 

 tive Ferns. 



The most peculiar member of the Family as regards its leaves is the 

 xerophytic Platyzoma, for it is heterophyllous (Fig. 482). The leaves are 

 densely crowded, and are inserted all round the creeping axis. The fertile 

 leaves are pinnate, resembling the simple leaves of the Eu-GleicJienia type. 

 They appear in successive crowded zones upon the growing plant which 



