172 HEREDITY AND EVOLUTION IN PLANTS 



Rubiaceae is the genus Mitchella, also relatively primitive, 

 and containing only two species, one in Japan, the other 

 the Partridge berry (M. repens) in North America. 



123. Evidence from the Distribution of Liverworts. — 

 The geographical distribution of the lower cryptogams 

 (below the ferns and their allies) has not been the subject 

 of as extensive study as that of the ferns and flowering 

 plants, but the evidence marshalled by Campbell^ in 1907, 

 concerning the distribution of the liverworts (Hepaticai), 

 illustrates in a striking manner the importance of the 



Fig. 78a. — Seed capsule and seeds of an orchid. 



facts of geographical distribution in endeavoring to 

 determine the question of the relative age of a group of 

 plants. It had been argued by Scott, in 1906, that the 

 liverworts were probably of comparatively recent origin 

 because of the almost entire absence of fossil remains in the 

 Paleozoic rocks. But, as Scott himself records, impres- 

 sions have been described from Paleozoic strata of plant 

 forms that can be assigned only to the Hepaticae, and 

 indeed to one of the most highly organized groups — the 



' Campbell, Douglas Houghton. On the distribution of the Heptaticae, 

 and its significance. New Phytologist 6 : 203-2 12. Oct. 1907. 



