BRYOPIIYTES 307 



The archesporium forms new cells, and the last ones 

 formed are mother cells, in each one of which four spores 

 are organized, the group of four being called a tetrad. 

 Among Bryophytes and the higher groups asexual spores 

 are always produced in tetrads. 



After the spores are formed the walls of the mother 

 cells disorganize, and the spores are left lying loose in 

 a cavity which was formerly occupied by the sporogenous 

 tissue. All mother cells do not always organize spores. 

 In some cases some of them are used up in supplying nour- 

 ishment to those which form spores. In other cases, certain 

 mother cells become much modified in form, being organ- 

 ized into elongated, spirally-banded cells called elaters (Fig. 

 286), meaning "drivers" or "hurlers." These elaters lie 

 among the loose ripe spores, are discharged with them, and 

 by their jerking movements assist in scattering them. 



The sporogonium is a very important structure from 

 the standpoint of evolution, for it represents the conspicu- 

 ous part of the higher plants. The "fern plant," and 

 the herbs, shrubs, and trees among " flowering plants," 

 correspond to the sporogonium of Bryophytes, and not to 

 the leafy branch (gametophope) or " moss plant." 



