VI] THREE LINES OF EVOLUTION 73 



formed into the aerial shoot directly, and was not built up 

 partly of decurrent branches. This was the case in the Spheno- 

 psida. On the other hand in the Pteropsida, and to a less general 

 extent in the Lycopsida, the erect axes possessed a pericaulome, 

 as Potonie^ has shown, the external tissues being morphologically 

 lateral organs fused to the original axis. The same thing of covirse 

 occiu's in many Algae, e.g. Polysiphonia. 



The leaves also vary in origin. In the Sphenopsida they are 

 metamorphosed small lateral branch systems, and, in the 

 Pteropsida, large lateral branch systems. In Lycopsida they are 

 metamorphosed emergences. 



We should define these lines of evolution as follows : 



(1) Sphenopsida, descended from Thallophytic Algae bearing 

 whorled branches. Limited, whorled branches typically small, 

 converted into leaves which were originally and always micro- 

 phyllous. Stem not built up of foliar decurrences. Sporangia and 

 sporangiophores, modifications of branches or segments of the 

 same. 



(2) Pteropsida, descended from Thallophytic Algae in which 

 the branches were large, munerous, scattered and not whorled. 

 Branches compound, eventually metamorphosed to mega- 

 phyllous leaves. Stem largely built up of foliar decurrences 

 (pericaulome). Sporangia and sporangiophores, modifications of 

 segments of branches. 



(3) Lycopsida, descended from Thallophytic Algae in which 

 the aerial axes were rarely branched and then usually in a 

 dichotomous manner. The branches bore microscopic or macro- 

 scopic emergences which were metamorphosed to microphyllous 

 leaves. Stem partly built up of foliar decurrences in some cases. 

 Sporangia and sporangiophores, modifications of emergences or 

 segments of emergences. 



The above outline agrees exactly with Potonie as regards 

 Algal ancestry, but to a limited extent only as regards peri- 

 caulome characters. It agrees with Lignier as regards the 



^ Potonie (1898), p. 19, (1902i), (19022). The pericaulome theory has more 

 recently been disputed by Kubart (1918) on anatomical grounds. In the 

 present paper, however, it is used solely as a morpholooical conception 

 without reference to the physiolooical functions which the fused organs 

 may be supposed to have performed originally. 



