Fig. 95 Desmarestia. A, plant with summer and winter appear- 

 ance ( X ^). B, apex showing cortication. C, transverse section 

 stipe. Dj female gametophyte. o = oogonium. E, male gametophyte. 

 a — antheridium, e = empty antheridium. F-J, stages in sporeling 

 germination. (A, after Newton; B, C, after Oltmanns; D-J, after 



Schreiber.) 



cortex, the cells of which become smaller toward the periphery. In 

 the older parts, small hyphal cells are interspersed in the cortex: 

 these originate as a result of secondary activity from the cortical 

 cells. Large hyphae, which have originated from the axial cells, 

 may form a system of veins within the cortex. 



The unilocular sporangia are on slightly raised portions of the 

 thallus and develop from cortical cells which undergo scarcely any 

 modification. Meiosis takes place in the sporangium, and the ripe 

 zoospores escape in a mass and germinate to give rise to dioecious 

 filamentous gametophytes which are heterothalHc. The smaller 

 male plants produce terminal antheridia from each of which is 

 liberated a single antherozoid, whilst the larger female plants pro- 

 duce the swollen oogonia. Each oogonium gives rise to a single 

 ovum which escapes, but as fertilization and germination take place 

 just outside the pore of the oogonium the young sporophyte de- 

 velops as far as the monosiphonous stage whilst still possessing a 

 primitive holdfast in the shape of the empty oogonium. Cortication, 

 which is best observed near the apex of old plants, commences in 

 the young plants after a few weeks, and further growth is main- 



169 



M 



