BRYOPHYTA 



169 



Each of these now divides by a vertical wall, followed by another at right 

 angles to it. Additional divisions, without definite sequence, take place 

 in all three planes. Then periclinal walls cut off an outer layer, the 

 amphithecium, from a central group of cells, the endothecium. As the 

 embryo continues to grow, the entire central group becomes sporogenous, 

 while the outer layer remains sterile. After the sporogenous cells have 

 divided for the last time, they separate and round off to become spore 

 mother cells (Fig. 137). Each of these then enlarges and undergoes two 



ABC 



Fig. 136. Development of the embryo of Riccia natans, X400. .*i, two-celled stage; B, 

 four-celled stage; C, later stage, showing differentiation into amphithecium and endo- 

 thecium. 



consecutive divisions during which the number of chromosomes is reduced 

 one-half, and a tetrad of cells is formed. The walls thicken and the four 

 members of the tetrad separate as mature spores. During the early 

 development of the sporophyte, the venter of the archegonium becomes 

 two-layered and forms the calyptra (Fig. 136). The sterile jacket of 

 the sporophyte and the inner layer of the calyptra break down before 

 the spores have ripened, leaving them enclosed within the outer laj^er 

 of the calyptra. Riccia has no spore-dispersing mechanism, the spores 

 being liberated by progressive decay of the thallus. 



In practically all the other Marchantiales the sporophyte consists of a 

 foot, seta, and a capsule containing both spores and elaters. As in Riccia, 

 the first division of the fertilized egg is transverse. In some genera, such 

 as Reboulia, Asterella, Conocephalum, and others, the next two divisions 

 are also transverse, resulting in a filament of four superimposed cells 

 (Fig. 138). Then vertical walls come in and, with the formation of 



