CHAPTER XV 



THE PTERIDOPHYTA : LYCOPODIALES, ISOETALES, 

 PSILOTALES AND EQUISETALES. THE CLUB 

 MOSSES AND HORSETAILS 



LYCOPSIDA 



This series of the Pteridophyta was distinguished by Jeffrey on the grounds 

 of the prevalence of small leaves, which have almost invariably a single 

 leaf-trace bundle, without branches. These traces leave no leaf gap in the 

 stele of the stem. They are described as microphyllous, partly on the 

 above grounds and partly because the leaf is subordinate to the axis in 

 the architecture of the shoots. 



It was pointed out, when speaking of the Pteropsida, that the morpho- 

 logical differences of the two series are not absolute, but that in spite of 

 exceptions their general characteristics do mark them as distinct. We might 

 add that the prevalent tendency to form reduced or saprophytic gametophytes 

 is another mark of distinction in the Lycopsida. 



The shoot of the Lycopsida often shows a tendency to be flattened by 

 the formation of side shoots all in one plane. The trace bundles which 

 supply these shoots leave a gap in the stele of the main axis which is similar 

 to the gap left by the megaphyllous leaves of Ferns. It has been repeatedly 

 suggested that the Fern megaphyll is homologous with one of these flattened 

 shoots in the Lycopsida and that it is therefore really a cladode (see p. 924) 

 or branch structure. From this point of view megaphylls and microphylls 

 must be regarded as two quite different types of organs, and if this be accepted 

 the distinction between the two series is made even more emphatic. Jeffrey 

 used the term cladosiphonic for the Lycopsidan type with only branch- 

 gaps in the stele, and phyllosiphonic for the Pteropsidan type with leaf- 

 gaps. 



The Lycopsida include two orders, of which we shall describe the follow- 

 ing examples : — 



Lycopodiales 



Lycopodiaceae (Homosporous), e.g., Lycopodhnn. 

 Selaginellaceae (Heterosporous), e.g., Selaginella. 



Isoetales 



Isoetaceae (Heterosporous), e.g., hoetes. 



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