Evolution of Plants 341 



ing protonemal stage, that intervenes between zygospore ger- 

 mination and the development of a sexual plant, is a very 

 constant feature in the Coleochsetacese, Hepaticse, Musci, 

 and even higher organisms; while the formation of the sexual 

 plant on it takes place either as a terminal or lateral outgrowth 

 from it. 



We would conclude then, from the above comparisons, that 

 the Hepaticse have directly originated from ancestral forms 

 that approach nearest to the Coleochaetacese of living fresh- 

 water algse. Lotzy has proposed the name Sphceroriccia for 

 a hypothetical intermediate type (119, 2: 88) that may have 

 combined characters of the Coleochsetacese, the Ricciacese, 

 and the Sphserocarpacese. The term "sphseroriccioid ances- 

 tor" would be preferable, for, while Sphcerocarpus shows nearer 

 approach in some details, Riccia shows points of closer affinity 

 in others. An exact generic term also for a hypothetical form 

 is inadvisable. 



It would be superfluous here to trace the genealogical rami- 

 fications of the Hepaticse further, but, in spite of their almost 

 complete absence as fossil remains, all evidence points to their 

 early origin, possibly during the middle period of the upper 

 archaean epoch. But increasing adaptation to a land life, 

 and continued action of subaerial stimuli, with proenvironal 

 response in varied ways by the organisms to such stimuli, 

 has resulted in evolution of the more complex and specialized 

 groups, such as the Metzgeriacese, the Anthocerotacese, the 

 Marchantiacese, and the highest or Jungermanniacese, though 

 none of these has advanced to the degree of complexity shown 

 by the leafy mosses. 



But in making such a statement it is well to bear in mind 

 that ancient and even greatly more comj^lex types may have 

 existed but may have become extinct, owing to the relative 

 softness of their tissues. For, had not the giant Calamites 

 and Lepidodendra of the coal period possessed hard and resist- 

 ing tissues, we might have inclined to the view that the humble 

 Equisetums and Lycopods of our existing flora represented 

 the highest and most complex types of their classes, a view 

 that would have been greatly wide of the mark. 



