BRYOPI1YTES 103 



does not follow that the archesporial cells themselves pro- 

 duce spores, but that the spores are to appear sooner or 

 later in their progeny. Usually the archesporial cells 

 divide and form a larger mass of spore-producing cells. 

 Such cells are known as sporogenous ("spore-producing") 

 cells, or the group is spoken of as sporogenous tissue. Spo- 

 rogenous cells may divide more or less, and the cells of the 

 last division are mother cells, those which directly produce 

 the spores. The usual sequence, therefore, is archesporial 

 cells (archesporium), sporogenous cells, and mother cells ; 

 but it must be remembered that they all may be referred 

 to as sporogenous cells. 



Each mother cell organizes within itself four spores, 

 the group being known as a tetrad. In 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 sporoge- 

 nous 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. Such mother cells are 

 said to function as nutritive cells. In other cases, certain 

 mother cells become much modified in form, being organ- 

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

 97, 101), 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 cells of the sporogonium which do not enter into 

 the formation of the archesporium, and are not sporoge- 

 nous, are said to be sterile, and are often spoken of as 

 sterile tissue. Every sporogonium, therefore, is made up 

 of sporogenous tissue and sterile tissue, and the differences 

 found among the sporogonia of Bryophytes depend upon 

 the relative display of these two tissues. 



The sporogonium is a very important structure from 

 the standpoint of evolution, for it represents the conspicu- 



