BRYOPHYTA 



165 



the apical cell and soon become sunken in the thallus by upgrowth of the 

 surrounding tissues, each coming to lie in an individual pit. 



In the other Marchantiales the antheridia are similarly sunken in pits 

 but the archegonia are not. The antheridia may be borne in irregular 

 median groups on the dorsal side of the thallus, as in some species of 

 Asterella, but more commonly they occur on a definite receptacle. This 

 may be cushion-like and sessile, asmReboidiasindConocephalum, or raised 

 above the thallus on a stalk, as in Marchantia. The antheridial recepta- 

 cle of Marchantia has a number of marginal growing points, from each 



Fig. 132. Male structures of Marchantia polymorpha. A, longitudinal section through 

 young male receptacle, showing embedded antheridia, X40; S, nearly mature antheridium, 

 X200; C, a single sperm, more highly magnified. 



of which an acropetal series of antheridia extends toward the center, the 

 antheridia being sunken in the upper surface of the receptacle (Figs. 

 13L4 and 132A). 



In Rehoulia, Asterella, Conocephalum, and many other genera the 

 archegonia are borne on a stalked receptacle that, with few exceptions, 

 is terminal in position and represents a specialized upright branch of the 

 thallus. Unlike the antheridia, the archegonia are not embedded in pits. 

 The female receptacle is commonly hemispherical or conical and more or 

 less lobed. Each lobe represents a separate growing point back of 

 which either one or several archegonia arise. As the receptacle grows, 

 the archegonia are carried to a position on its lower side close to the stalk. 

 In Marchantia the archegonial receptacle reaches its greatest degree of 

 specialization. It does not have lobes, but consists of a number of rays 

 alternating with groups of archegonia (Figs. 13 IB and 133.4). The 

 archegonia hang with the necks downward. 



