292 COLLEGE BOTANY 



and produce new gametophytes. The complete life history is 

 illustrated in Fig. 141. 



The genus Riccia, also belongs to the Marchantiales and con- 

 tains a number of species, some of which are terrestrial and 

 others aquatic. They are smaller than M. polymorpha and the 

 antheridia and archegonia remain embedded in cavities of the 

 thallus. They are the simplest of -the Hepaticce. 



The Jungermanniales contains a larger .number of species 

 than either of the other two groups. They grow under conditions 

 ranging from the extremely wet to the extremely dry. They are 



most abundant in the tropics and 

 grow most commonly on the ground, 

 on tree trunks and oil leaves. They 

 are divided into two groups, the thai- 

 lose forms, which resemble the Mar- 

 chantiales, and the foliose forms, 

 which resemble the mosses, but there 

 is a gradual gradation between the 

 two groups. However, the distinc- 

 tive characters of the Junnermanni- 



FIG. 140. (a) female plant of . f . ' 



Marchantia polymorpha bearing two dies IS fa) be f OUnd in the archegonia 

 archegonial branches; (6) also a 



single antheridial branch from a and the SpOrOphyte. 

 male plant. ^^ 



The Anthocerotales is a small> 



temperate zone group, which is thought by many workers to 

 represent the ancestral forms of the Pteridopliytes. We cannot 

 discuss this order in a limited work of this kind. 



It will be noted that in the Hep&twce the gametophyte is 

 much larger than the sporophyte and that there are no true roots, 

 stems or leaves. There is a differentiation of the cell structure 

 but no collenchyma, sclerenchyma, fibrous, tracheary, sieve or 

 laticiferous tissues. 



The Musci or mosses are also a very old group of plants which 

 are more highly differentiated than the Hepaticce. They are 

 widely distributed and were much more abundant during the 

 carboniferous age of the earth's history than at the present time. 



