﻿The genus Plagiochasma and its North American species * 



Historical Introduction 



Of the various Marchantiaceae occurring in North America the 

 genus Plagiochasma has long been in especial need of critical study 

 and revision. It is hoped that the present paper may clear up some 

 of the confusion and misunderstanding regarding it, although the 

 writer admits that further work will be necessary before our knowl- 

 edge of the species can be considered at all complete. Prolonged 

 observation in the field under varying conditions of light and 

 moisture is especially to be recommended, while further collection 

 of material, particularly in fruiting condition, is much to be desired. 

 Fortunately dried specimens are more satisfactory for study than 

 in most genera of the thallose liverworts, even if they can never 

 take the place of material properly fixed and preserved. 



The genus Plagiochasma Lehm. & Lindenb., as originally pro- 

 posed by its authors in 1832 (17, p. 13), contained two undescribed 

 species from Nepal, P. cordatum and P. appendiculatum. The first 

 of these should be regarded as the type of the genus. Four years 

 later Lehmann (16) published another genus, Anihrocephalus, which 

 was likewise based on an undescribed species from Nepal, A. 

 nepalensis. In 1838 Nees von Esenbeck (26, pp. 33, 40) accepted 

 both of these genera. He pointed out the fact, however, that 

 Ay Ionia rupestris Forst. was referable to Plagiochasma, and he 

 added a new species, P. peruvianum Nees & Mont., to this genus. 

 Aytonia rupestris, which was published in 1776, was based on 

 material collected by its authors on the island of Madeira, but 

 Nees von Esenbeck was able to extend its known range to the 

 island of Corfu, in Greece, citing specimens collected by Spathys. 

 Even at the time he wrote the species had received many names, 

 but he discarded them all and renamed the plant P. Aitonia 

 Lindenb. & Nees, 



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