414 BRYOPHYTES (MOSS PLANTS) 



somes are so distributed that each daughter cell gets only half 

 of the number of chromosomes or the gametophytic number. 

 This kind of cell division is called the reduction division and 

 simply undoes the doubling of chromosomes resulting from fer- 

 tilization. The diagrams in Figure 368 show how the reduction 

 division differs from ordinary cell division. Cytologically the 

 sporophyte begins with the fertilized egg and ends with mother 

 cells, while the gametophyte begins with the spore and ends with 

 fertilization. More will be said about the significance of the 

 reduction division in connection with heredity where it has an 

 important bearing. 



The Riccias. — The genus Riccia, which is often regarded as a 

 subdivision of the Marchantiales, includes the simplest of Liver- 

 worts. Some of them are almost entirely aquatic, living sub- 



Fia. 369. — One of the Riccias, the simplest of Liverworts. X 4. 



merged or floating on the surface of the water, while others live 

 spread out on moist soil. The plant body is a simple thallus, 

 smaller and not so well differentiated as the thallus of Marchantia. 

 {Fig. 369.) No gametophores are developed and the sex organs, 

 both kinds of which may develop on the same plant, occur in 

 grooves along the ribs of the thallus. The air pores are not well 

 developed and sometimes rhizoids are absent. The sporophyte, 

 which is also much simpler than the sporophyte of the Mar- 

 chantias, lacks a foot and stalk, and thus consists of only a 

 sporangium. 



When the sporophytes of the Riccias and Marchantias are 

 compared, it is obvious that much more of the fertilized egg has 

 been turned into spores in the Riccias than in the Marchantias. 

 In the Marchantias much of the cell progeny of the fertilized egg, 

 instead of forming spores, is used in forming a foot, stalk, and 

 elaters. Such a diverting of the cells which could form spores 



