LIVERWORTS 



34i 



the egg case (fig. 301), the capsule opens and the spores are 

 scattered. The spores germinate under favorable conditions and 

 produce the thallus of the marchantia, thus completing the life 

 cycle. 



492. Riccia. Riccia is another of the thallose liverworts. 

 The plants are thin and leaf-like, some being nearly circular, 

 others semicircular, others narrowly strap-shaped. They branch 

 in a forked manner so that the 



circular forms are split or indented 

 inward from the edge, while some 

 of the strap-shaped forms produce 

 rosettes. They grow on wet ground 

 or float on the water. There are no 

 special outgrowths on which the 

 sexual organs are borne. The sperm 

 and egg cases are developed in the 

 upper side of the thallus along the 

 middle line. The form of the sexual 

 organs is very similar to those of 

 Marchantia. The capsule, however, 

 is very different. It is a simple glo- Fig. 302. 



j , Elater and spore of Marchantia. 



bose capsule or spore case, developed s p t S p re ; me, mother ceil of spores, 



P . , . ,i i showing partly formed spores. 



from the egg within the enlarging 



venter of the egg case. The outer layer of cells is sterile, while 

 the inner portion forms nothing but spores, four spores being 

 formed from a single mother cell as in Marchantia. All of the 

 mother cells in Riccia form spores, while in Marchantia some of 

 them form sterile cells, the elaters. Marchantia and Riccia each 

 represent a group of the thallose liverworts including many other 

 closely related forms. 



493. The foliose liverworts. These are the leafy-stemmed 

 liverworts. Each plant is really a thallus, but is highly special- 

 ized by the formation of a distinct slender axis (the stem) and 

 thin foliar organs (the leaves). This has probably come about 

 by a lobing of the margins of thallose forms, which became more 

 and more marked and specialized until these leafy-stemmed forms 



