172 



PLANT MORPHOLOGY 



mother cells (Fig. 140). As in Riccia, tetrads are formed and the walls 

 of the spores thicken. The elaters are long, slender cells, pointed at 

 each end, their walls developing spiral thickenings as the protoplasm 

 disappears (Fig. 140D). Elaters are hygroscopic and perform squirming 

 movements that assist in the liberation of the spores. The capsule wall 

 ■ one layer of cells thick except in the apical region. In RebouUa, 



IS 







M /"^Z^ 



Fig. 140. Sporophyte of Marchantia polymorpha. A, longitudinal section of nearly 

 mature sporophyte, showing the foot (/), seta (s), capsule (c), and ruptured calyptra (a); 

 B, two rows of spore mother cells and portion of an undeveloped elater; C, a row of spore 

 tetrads; D, three mature spores and the end of an elater; A, XQO; B, C, D, X600. 



Asterella, and related forms local thickenings are not formed on the cells 

 of the capsule wall and dehiscence takes place by means of an apical lid. 

 In nearly all the other genera, however, the cells of the capsule wall bear 

 annular thickenings, dehiscence occurring by irregular clefts. 



Summary. The Marchantiales are a group in which the gametophyte, 

 while remaining simple in form, has achieved a high degree of structural 

 complexity. In all members of the order the gametophyte is thalloid and 

 grows by means of a cuneate apical cell. It is nearly always differentiated 

 into an upper epidermis with air pores, a dorsal photosynthetic region 



