178 THE MORPHOLOGY OF PTERIDOPHYTES 



shown in Fig. 28). While these three trends are in the direc- 

 tion of progressive elaboration, the fourth is in the opposite 

 direction ; reduction is supposed to have brought about the 

 evolution of the simple unbranched microphyll of the 

 Lycopsida. The fifth trend, re-curving, is found in several 

 groups of plants, where the sporangiophore becomes re- 

 flexed and the sporangium inverted, as in anatropous ovules. 



Figs. 28H-K illustrate the way in which the sporangio- 

 phore might have evolved in the Sphenopsida. Here, re- 

 curving and syngenesis are the chief trends, resulting in a 

 peltate structure with reflexed sporangia. In the evolution of 

 the leaf of the Sphenopsida, however, the chief trends have 

 been planation, followed by reduction. Examples of inter- 

 mediate types existed among fossil members of the group : 

 Calamophyton, Hyenia, Eviostachya and Protocalamostachys 

 represented stages in sporangiophore evolution, while 

 Calamophyton and Asterocalamites provide stages in the 

 evolution of leaves. The Telome Theory, therefore, gives a 

 satisfactory explanation of the evolution of leaves, and of 

 sporangiophores, in this group. However, the growth habit 

 of the earliest members (e.g. Calamophyton, Protohyenia and 

 Hyenia) was a long way from the theoretical ancestral type. 



In the Pteropsida (Figs. 28L-O), planation, overtopping 

 and webbing have combined to produce the sterile and fertile 

 fronds of modern ferns. The fossil record provides abundant 

 examples of intermediate types of frond form (e.g. Pseudo- 

 sporochnus, Stauropteris, Botryopteris) but, again, the 

 growth habit of the earUest members was far removed from 

 the ancestral type postulated by the Telome Theory (in fact, 

 Cladoxylon was superficially very similar to Calamophyton). 



In the Lycopsida (Figs. 28P-S), the chief trend is supposed 

 to have been reduction. The bifid tips of the sporophylls and 

 leaves of Protolepidodendron may be brought forward in 

 support of this suggestion, but otherwise the fossil record 

 lacks good examples of intermediate types. The microphyll 

 had almost completed its evolution by the time the group 



